Unexpected git diff output during merge conflict

From: Nick O'Leary <nick.oleary@gmail.com>
Date: Feb 8, 2018, 10:51 UTC
Message-ID: <CAF=vhqdJ2fByjSVt37b34wQUFo5_tb7rHbCddsaCzdvKywSBdg@mail.gmail.com>

Hi,

I have a merge conflict on a file and the git diff output looks wrong to me.

Here's how to recreate:

On branch 'dev' add/commit a file (called README.md) with the contents ( '------' used to delimit the file, not included in the content):

------- This is my default readme -------

On branch 'master' add/commit the same file with the contents:

------- merged-history-test ===================

### About

This is your project's README.md file. It helps users understand what your project does, how to use it and anything else they may need to know. -------

Then, whilst on master, run `git merge dev` and get the inevitable merge conflict on this file.

Running `git diff README.md` gives:

$ git diff README.md diff --cc README.md index 61d78b2,620d806..0000000

--- a/README.md
+++ b/README.md
@@@ -1,7 -1,1 +1,11 @@@
 -This is my default readme
++<<<<<<< HEAD
 +merged-history-test
 +===================
 +
 +### About
 +
 +This is your project's README.md file. It helps users understand what your
- project does, how to use it and anything else they may need to know.
++project does, how to use it and anything else they may need to know.
++=======
++This is my default readme
++>>>>>>> dev

This does not look right to me. The 'This is my default readme' line
has ++ at the start - suggesting its new to both parent copies of the
file, which isn't the case - it came from the dev branch so should be
prefixed with '+ '.
I'm also not clear why the line beginning 'project does' has both a -
and ++ prefix.

In every description of the Combined Diff format that I've been able
to find, they only show having '+ ' or ' +' prefixes on the actual
content and '++' on the <<<</=====/>>>> lines.


Am I missing something here?

Thanks,
Nick