Re: [PATCH] rebase --skip: fix commit message clean up when skipping squash

> From: Phillip Wood <phillip.wood@dunelm.org.uk>
> 
> During a series of "fixup" and/or "squash" commands, the interactive
> rebase accumulates a commit message from all the commits that are being
> squashed together. If one of the commits has conflicts when it is picked
> and the user chooses to skip that commit then we need to remove that
> commit's message from accumulated messages.  To do this 15ef69314d5
> (rebase --skip: clean up commit message after a failed fixup/squash,
> 2018-04-27) updated commit_staged_changes() to reset the accumulated
> message to the commit message of HEAD (which does not contain the
> message from the skipped commit) when the last command was "fixup" or
> "squash" and there are no staged changes. Unfortunately the code to do
> this contains two bugs.
> 
> (1) If parse_head() fails we pass an invalid pointer to
>      unuse_commit_buffer().
> 
> (2) The reconstructed message uses the entire commit buffer from HEAD
>      including the headers, rather than just the commit message.
> 
> The fist issue is fixed by splitting up the "if" condition into several