Re: [PATCH 2/2] rebase -i: fix SIGSEGV when 'merge <branch>' fails

> From: Phillip Wood <phillip.wood@dunelm.org.uk>
> 
> If a merge command in the todo list specifies just a branch to merge
> with no -C/-c argument then item->commit is NULL. This means that if
> there are merge conflicts error_with_patch() is passed a NULL commit
> which causes a segmentation fault when make_patch() tries to look it up.
> 
> This commit implements a minimal fix which fixes the crash and allows
> the user to successfully commit a conflict resolution with 'git rebase
> --continue'. It does not write .git/rebase-merge/patch,
> .git/rebase-merge/stopped-sha or update REBASE_HEAD. To sensibly get the
> hashes of the merge parents would require refactoring do_merge() to
> extract the code that parses the merge parents into a separate function
> which error_with_patch() could then use to write the parents into the
> stopped-sha file. To create meaningful output make_patch() and 'git
> rebase --show-current-patch' would also need to be modified to diff the
> merge parent and merge base in this case.
> 
> Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>