receive-pack: factor out capability string generation
receive-pack: factor out capability string generation

Similar to the previous one for send-pack, make it easier and
cleaner to add to capability advertisement.

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