[PATCH v2 0/4] Fix ORIG_HEAD behavior of the built-in rebase

It was reported by Nazri Ramliy that ORIG_HEAD was set incorrectly again, this time caused by the shortcut to call git am directly, without detouring to a Unix shell script.

Patch 2/4 might look like something completely unrelated, but without it, the update to HEAD might use an incorrect reflog message.

Patch 1/4 is more a "while at it" patch; while looking which code paths might need to update ORIG_HEAD, I noticed that this reset_head() call did unnecessary work.

Changes since v1:

 * Reworded the unclear commit message of patch 1/4.

Johannes Schindelin (4):
  built-in rebase: no need to check out `onto` twice
  built-in rebase: use the correct reflog when switching branches
  built-in rebase: demonstrate that ORIG_HEAD is not set correctly
  built-in rebase: set ORIG_HEAD just once, before the rebase

 builtin/rebase.c  | 37 +++++++++++++++++++++----------------
 t/t3400-rebase.sh |  8 ++++++++
 2 files changed, 29 insertions(+), 16 deletions(-)

base-commit: 21853626eac565dd42572d90724b29863f61eb3b
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-153%2Fdscho%2Frebase-am-and-orig-head-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-153/dscho/rebase-am-and-orig-head-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/153

Range-diff vs v1:

 1:  2d99429387 ! 1:  59c3266ed5 built-in rebase: no need to check out `onto` twice
     @@ -4,12 +4,21 @@
      
          In the case that the rebase boils down to a fast-forward, the built-in
          rebase reset the working tree twice: once to start the rebase at `onto`,
     -    then realizing that the original HEAD was an ancestor, `reset_head()`
     -    was called to update the original ref and to point HEAD back to it.
     +    then realizing that the original (pre-rebase) HEAD was an ancestor and
     +    we basically already fast-forwarded to the post-rebase HEAD,
     +    `reset_head()` was called to update the original ref and to point HEAD
     +    back to it.
      
          That second `reset_head()` call does not need to touch the working tree,
     -    though, as it does not change the actual tip commit. So let's avoid that
     -    unnecessary work.
     +    though, as it does not change the actual tip commit (and therefore the
     +    working tree should stay unchanged anyway): only the ref needs to be
     +    updated (because the rebase detached the HEAD, and we want to go back to
     +    the branch on which the rebase was started).
     +
     +    But that second `reset_head()` was called without the flag to leave the
     +    working tree alone (the reason: when that call was introduced, that flag
     +    was not yet even thought of). Let's avoid that unnecessary work by
     +    passing that flag.
      
          Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
      
 2:  b92d76065d = 2:  67e41c032a built-in rebase: use the correct reflog when switching branches
 3:  1bc3cbad26 = 3:  17737998f4 built-in rebase: demonstrate that ORIG_HEAD is not set correctly
 4:  1786c172ef = 4:  7891c05f51 built-in rebase: set ORIG_HEAD just once, before the rebase