Correct usages of sed in git-tag for Mac OS X

Correct usages of sed in git-tag for Mac OS X

Both `git-tag -l` and `git tag -v` fail on Mac OS X due to their
non-standard uses of sed.  Actually `git tag -v` fails because the
underlying git-tag-verify uses a non-standard sed command.

We now stick to only standard sed, which does make our sed scripts
slightly more complicated, but we can actually list tags with more
than 0 lines of additional context and we can verify signed tags
with gpg.  These major Git functions are much more important than
saving two or three lines of a simple sed script.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>