t5551: factor out tag creation
t5551: factor out tag creation

One of our tests in t5551 creates a large number of tags,
and jumps through some hoops to do it efficiently. Let's
factor that out into a function so we can make other similar
tests.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git