pathspec: drop "lone : means no pathspec" from get_pathspec()

We may want to give the pathspec subsystem such a feature, but not while
we are still using get_pathspec() that returns a stupid "char **" that
loses subtle nuances that existed in the input string.

In the meantime, the callers of get_pathspec() that want to support it
could do an equivalent before feeding their argv[] to the function
themselves quite easily.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -197,9 +197,6 @@ const char *prefix_pathspec(const char *prefix, int prefixlen, const char *elt)
 		}
 		if (*copyfrom == ')')
 			copyfrom++;
-	} else if (!elt[1]) {
-		/* Just ':' -- no element! */
-		return NULL;
 	} else {
 		/* shorthand */
 		for (copyfrom = elt + 1;