Don't echo cmd if no substitutions were performed

Previously, git-number would always echo the command that it ran, even
if it did not substitute any numbers from the arguments. Now, git-number
will not echo the command that it ran if no number substitutions were
performed.

If a user wanted to pass lots of relevant git commands through
git-number first via shell aliases, then echoing what happened if
nothing happened is a bit bothersome.