dir.c: avoid "exceeds maximum object size" error with GCC v12.x

Technically, the pointer difference `end - start` _could_ be negative,
and when cast to an (unsigned) `size_t` that would cause problems. In
this instance, the symptom is:

dir.c: In function 'git_url_basename':
dir.c:3087:13: error: 'memchr' specified bound [9223372036854775808, 0]
       exceeds maximum object size 9223372036854775807
       [-Werror=stringop-overread]
    CC ewah/bitmap.o
 3087 |         if (memchr(start, '/', end - start) == NULL
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

While it is a bit far-fetched to think that `end` (which is defined as
`repo + strlen(repo)`) and `start` (which starts at `repo` and never
steps beyond the NUL terminator) could result in such a negative
difference, GCC has no way of knowing that.

See also https://gcc.gnu.org/bugzilla//show_bug.cgi?id=85783.

Let's just add a safety check, primarily for GCC's benefit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -3076,6 +3076,15 @@ char *git_url_basename(const char *repo, int is_bundle, int is_bare)
 			end--;
 	}
 
+	/*
+	 * It should not be possible to overflow `ptrdiff_t` by passing in an
+	 * insanely long URL, but GCC does not know that and will complain
+	 * without this check.
+	 */
+	if (end - start < 0)
+		die(_("No directory name could be guessed.\n"
+		      "Please specify a directory on the command line"));
+
 	/*
 	 * Strip trailing port number if we've got only a
 	 * hostname (that is, there is no dir separator but a