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 <email@example.com>