contrib: git-c2t converts tag creating commits to tags
Introduce git-c2t <commit> <tag> which converts a commit object <commit>
into a tag object (and tags it with <tagname>), under the provision that
<commit> has exactly one parent and has the same tree. That tag points
at the parent commit.
This is useful for converting "tag creating commits" such as those
produced by git-svn into proper git tags. Tag creating commits typically
create a "1-commit side branch" whereas the tag created by c2t points at
the pseudo fork point (the commit being tagged).
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>