git-gui: Don't kill modified commit message buffer with merge templates

git-gui: Don't kill modified commit message buffer with merge templates

If the user is in the middle of a merge and has already started to
modify their commit message we were losing the user's changes when
they pressed 'Rescan' after resolving issues or making changes in
the working directory.

The problem here was our background timer that saves the commit
message buffer.  It marks the commit message buffer as not being
modified when it writes it out to disk, so during the rescan we
assumed the buffer should be replaced with what we read from the
MERGE_MSG file.  So we now only read these files from .git if we
have a valid backup file.  Since we clear it on commit this will
only have an impact while the user is actively editing the current
commit.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>