cherry-pick: make sure all input objects are commits

cherry-pick: make sure all input objects are commits

When a single argument was a non-commit, the error message used to be:

	fatal: BUG: expected exactly one commit from walk

For multiple arguments, when none of the arguments was a commit, the error was:

	fatal: empty commit set passed

Finally, when some of the arguments were non-commits, we ignored those
arguments.  Fix this bug and make sure all arguments are commits, and
for the first non-commit, error out with:

	fatal: <name>: Can't cherry-pick a <type>

Signed-off-by: Miklos Vajna <vmiklos@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>