Add time-stamping functionality to git verify-tag

This commit introduces command line options for git verify-tag to allow
verification of RFC3161 time-stamped tags.

To keep consistent with the current behavior of verifying gpg signatures, the
return value of `git verify-tag` still indicates only the success of gpg
signature verification by default. To influence this behavior, the configuration
variable `ts.failonverify` is introduced.

ts.failonverify set to 0: use default behavior
	Return 1 if gpg verification failed or no signature was found
	Return 0 on success.

If config variable ts.failonverify is set, the return value includes the status
of time-stamp verification in the second lowest bit. Possible return values are:
	0 - both gpg and time stamp verification succeeded
	1 - gpg failed, time stamp succeeded
	2 - gpg succeeded, time stamp failed
	3 - both gpg and time stamp verification failed

Command line parameters `-s` or `-t` override the configuration
variable mentioned above.

Signed-off-by: Anton Würfel <anton.wuerfel@fau.de>
Signed-off-by: Phillip Raffeck <phillip.raffeck@fau.de>