git-gui: Allow git-merge to use branch names in conflict markers

git-gui: Allow git-merge to use branch names in conflict markers

Earlier when I rewrote the merge implementation for git-gui I broke
it such that the conflict markers for the "theirs" side of the hunk
was using a full SHA-1 ID in hex, rather than the name of the branch
the user had merged.  This was because I got paranoid and passed off
the full SHA-1 to git-merge, instead of giving it the reference name
the user saw in the merge dialog.

I'd still like to resolve the SHA-1 upfront in git-gui and always use
that value throughout the merge, but I can't do that until we have a
full implementation of git-merge written in Tcl.  Until then its more
important that the conflict markers be useful to the end-user, so we
need to pass off the ref name and not the SHA-1 ID.

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