New tests and en-passant modifications to mktag.

New tests and en-passant modifications to mktag.

These changes were originally part of the next patch, but have been
split out since they were peripheral to the main purpose of that patch.

  - update comment describing the signature format to reflect
    the current code.
  - remove trailing \n in calls to error(), since a \n is already
    provided by error().
  - remove redundant call to get_sha1_hex().
  - call sha1_to_hex(sha1) to convert to ascii, rather than attempting
    to print the raw sha1.

The new tests provide a regression suite to support the modifications
to git-mktag in this and the next patch.

Signed-off-by: Ramsay Allan Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <junkio@cox.net>