Re: [BUG] [git 2.16.1] yeeek ... my files are gone .. by git pull <otherRepositoryUrl>

1. steps to reproduce
=====================
```
Last login: Fri Feb 23 00:33:11 on ttys001
~ PATH variable not enhanced, no applications found in ~/Applications/*-latest

-bash:/Users/marcel:$ mkdir /tmp/$$ change to new directory '/tmp/2608'? [Y/n]

-bash:/tmp/2608:$ mkdir a.git change to new directory 'a.git'? [Y/n]

-bash:/tmp/2608/a.git:$ git init Initialized empty Git repository in /private/tmp/2608/a.git/.git/

-bash:/tmp/2608/a.git:$ touch foo

-bash:/tmp/2608/a.git:$ git add foo

-bash:/tmp/2608/a.git:$ git commit -m "foo" foo
[master (root-commit) ed191c4] foo
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 foo

-bash:/tmp/2608/a.git:$ cd - /tmp/2608

-bash:/tmp/2608:$ mkdir b.git change to new directory 'b.git'? [Y/n]

-bash:/tmp/2608/b.git:$ git init Initialized empty Git repository in /private/tmp/2608/b.git/.git/

-bash:/tmp/2608/b.git:$ touch bar

-bash:/tmp/2608/b.git:$ git add bar

-bash:/tmp/2608/b.git:$ git commit -m "bar" bar
[master (root-commit) 80b0355] bar
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 bar

-bash:/tmp/2608/b.git:$ cd - /tmp/2608

-bash:/tmp/2608:$ git clone a.git c Cloning into 'c'... done.

-bash:/tmp/2608:$ cd c

-bash:/tmp/2608/c:$ ll total 0 drwxr-xr-x 12 marcel wheel 384B 23 Feb 05:47 .git -rw-r--r-- 1 marcel wheel 0B 23 Feb 05:47 foo

-bash:/tmp/2608/c:$ git pull ../b.git/
warning: no common commits
remote: Counting objects: 3, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From ../b
 * branch            HEAD       -> FETCH_HEAD
Successfully rebased and updated refs/heads/master.

-bash:/tmp/2608/c:$ ll total 0 drwxr-xr-x 14 marcel wheel 448B 23 Feb 05:47 .git -rw-r--r-- 1 marcel wheel 0B 23 Feb 05:47 bar

-bash:/tmp/2608/c:$ git reflog 80b0355 (HEAD -> master) HEAD@{0}: pull ../b.git/: checkout 80b03552466bc526b1130ce5ca4a991ba31a0546: returning to refs/heads/master 80b0355 (HEAD -> master) HEAD@{1}: pull ../b.git/: checkout 80b03552466bc526b1130ce5ca4a991ba31a0546 ed191c4 (origin/master, origin/HEAD) HEAD@{2}: clone: from /tmp/2608/a.git

-bash:/tmp/2608/c:$ git remote -v origin /tmp/2608/a.git (fetch) origin /tmp/2608/a.git (push)

-bash:/tmp/2608/c:$  git log --all --graph --decorate --oneline --simplify-by-decoration
* 80b0355 (HEAD -> master) bar
* ed191c4 (origin/master, origin/HEAD) foo
```

2. expected result
==================
just an error in case the too trees have no common ancestors

```
-bash:/tmp/2608/c:$ git pull ../b.git/
warning: no common commits
remote: Counting objects: 3, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From ../b
 * branch            HEAD       -> FETCH_HEAD
fatal: refusing to merge unrelated histories
```

3. actual result
================
pulls out, removes all files from the first tree

4. the difference and why it was unexpected
===========================================
I can’t find words on it … it should not work but it did? somehow … with unexpected results to my local repository

it somehow seems to be an issue of my config, because resetting it, will not allow the pull as expected

``` -bash:/tmp/2608/c:$ GIT_CONFIG_NOSYSTEM=1 HOME=. git config -l core.repositoryformatversion=0 core.filemode=true core.bare=false core.logallrefupdates=true core.ignorecase=true core.precomposeunicode=true remote.origin.url=/tmp/2608/a.git remote.origin.fetch=+refs/heads/*:refs/remotes/origin/* branch.master.remote=origin branch.master.merge=refs/heads/master

-bash:/tmp/2608/c:$ GIT_CONFIG_NOSYSTEM=1 HOME=. git pull ../b.git/
warning: no common commits
remote: Counting objects: 3, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From ../b
 * branch            HEAD       -> FETCH_HEAD
fatal: refusing to merge unrelated histories

-bash:/tmp/2608/c:$ git pull ../b.git/
From ../b
 * branch            HEAD       -> FETCH_HEAD
Successfully rebased and updated refs/heads/master.
```

the logs tells me he rebases ... ``` -bash:/tmp/2608/c:$ git config -l | grep merge diff.tool=p4merge merge.tool=p4merge merge.branchdesc=true merge.log=true branch.autosetupmerge=true branch.master.merge=refs/heads/master

-bash:/tmp/2608/c:$ git config -l | grep pull pull.rebase=preserve

-bash:/tmp/2608/c:$ git config -l | grep fetch fetch.recursesubmodules=true remote.origin.fetch=+refs/heads/*:refs/remotes/origin/* ```

I’m sorry, I just wanted to write down first any helpful information without - being late - having time to go into further investigations myself … hopefully to get some answers that „this is ok, you’re just stupid and didn’t read the spec/doc“ or „ok, this seems strange to me too, can we go forward in details and analysis“ ;)

```
-bash:/tmp/2608/c:$ git status
On branch master
Your branch and 'origin/master' have diverged,
and have 1 and 1 different commits each, respectively.
  (use "git pull" to merge the remote branch into yours)

nothing to commit, working tree clean

-bash:/tmp/2608/c:$ git branch -avv
* master                80b0355 [origin/master: ahead 1, behind 1] bar
  remotes/origin/HEAD   -> origin/master
  remotes/origin/master ed191c4 foo

```

just to say: the latter „refused pull“ is what I expect the first time too!

SUSPECTED / PROVEN to found a guilty setting
=============================================
```
-bash:/tmp/2608/c:$ GIT_CONFIG_NOSYSTEM=1 HOME=. git -c user.name="me" -c user.email=me@example.com pull -r ../b.git/
warning: no common commits
remote: Counting objects: 3, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From ../b
 * branch            HEAD       -> FETCH_HEAD
First, rewinding head to replay your work on top of it...
Applying: foo

-bash:/tmp/2608/c:$ ll total 0 drwxr-xr-x 14 marcel wheel 448B 23 Feb 06:21 .git -rw-r--r-- 1 marcel wheel 0B 23 Feb 06:21 bar -rw-r--r-- 1 marcel wheel 0B 23 Feb 06:21 foo

``` shows me a quite different behavior, so solely rebase not seems the full problem BUT `--rebase=preserve` will .. o’man , really, is this intended?

Yes, I see, rebase is a harmful operation and yes, you might now tell me a fool to apply this per default

but why is this unrelated history check different from pull (merge) to rebase to rebase -p ? Can’t find a word on it https://git-scm.com/docs/git-rebase#git-rebase---preserve-merges

With regards, ~Marcel