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: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -491,6 +491,7 @@ case "$#" in
 	then
 		head_name="detached HEAD"
 	else
+		echo >&2 "fatal: no such branch: $1"
 		usage
 	fi
 	;;