Make mktag a builtin

This series makes mktag a builtin and unifies the verification of
created tags between git-tag and git-mktag.

 Makefile      |    2 +-
 builtin-tag.c |  183 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 builtin.h     |    1 +
 git.c         |    1 +
 mktag.c       |  180 --------------------------------------------------------
 5 files changed, 186 insertions(+), 181 deletions(-)

-brandon