git-gui: Permit merging tags into the current branch.

git-gui: Permit merging tags into the current branch.

It was pointed out on the git mailing list by Martin Koegler that
we did not show tags as possible things to merge into the current
branch.  They actually are, and core Git's Grand Unified Merge
Driver will accept them just like any other commit.

So our merge dialog now requests all refs/heads, refs/remotes and
refs/tags named refs and attempts to match them against the commits
not in HEAD.  One complicating factor here is that we must use the
%(*objectname) field when talking about an annotated tag, as they
will not appear in the output of rev-list.

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