rebase: give a better error message for bogus branch
rebase: give a better error message for bogus branch

When you give a non-existent branch to git-rebase, it spits
out the usage. This can be confusing, since you may
understand the usage just fine, but simply have made a
mistake in the branch name.

Before:

  $ git rebase origin bogus
  Usage: git rebase ...

After:

  $ git rebase origin bogus
  fatal: no such branch: bogus
  Usage: git rebase ...

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