Johannes Sixt pointed out that git-gui was randomly selecting which branch (or tag!) it will show in the merge dialog when more than one ref points at the same commit. This can be a problem for the user if they want to merge a branch, but the ref that git-gui selected to display was actually a tag that points at the commit at the tip of that branch. Since the user is looking for the branch, and not the tag, its confusing to not find it, and worse, merging the tag causes git-merge to generate a different message than if the branch was selected.
While I am in here and am messing around I have changed the for-each-ref usage to take advantage of its --tcl formatting, and to fetch the subject line of the commit (or tag) we are looking at. This way we could present the subject line in the UI to the user, given them an even better chance to select the correct branch.
Signed-off-by: Shawn O. Pearce <email@example.com>