Dgit.pm: resolve_upstream_version: Report tag, where sensible

Dgit.pm: resolve_upstream_version: Report tag, where sensible

This affect what is show in commit messages (including the parseable
parts) made by dgit and git-debrebase.  Now they prefer to refer to
the tag name if the caller specified refs/tags/ and the tag name is
a nice one.

This is not expected to make much difference for human callers (who
will probably not explicitly qualify their tag refs).  But it will
make a difference for scripts.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>