Make git tag a builtin.

Make git tag a builtin.

This replaces the script "git-tag.sh" with "builtin-tag.c".

The existing test suite for "git tag" guarantees the compatibility
with the features provided by the script version.

There are some minor changes in the behaviour of "git tag" here:
"git tag -v" now can get more than one tag to verify, like "git tag -d" does,
"git tag" with no arguments prints all tags, more like "git branch" does,
and "git tag -n" also prints all tags with annotations (without needing -l).
Tests and documentation were also updated to reflect these changes.

The program is currently calling the script "git verify-tag" for verify.
This can be changed porting it to C and calling its functions directly
from builtin-tag.c.

Signed-off-by: Carlos Rica <jasampler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>