tg-tag: prefer refs/heads disambiguations
While `tg revert` deals mainly in `tag` objects typically located
under `refs/tags/...`, `tg tag` deals with branches typically located
under `refs/heads/...`.
Instead of complaining and dying when a branch and tag with the
same name exist and the "short" branch name is given, go ahead and
prefer the `refs/heads/...` interpretation if that's an existing
ref name. This is almost always the correct DWIM interpretation.
Also update the documentation to make this DWIM interpretation clear.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>