real_path(): properly handle nonexistent top-level paths

The change has two points:

1. Do not strip off a leading slash, because that erroneously turns an
   absolute path into a relative path.

2. Do not remove slashes from groups of multiple slashes; instead let
   chdir() handle them.  It could be, for example, that it wants to
   leave leading double-slashes alone.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

abspath.c

@@ -45,8 +45,8 @@ const char *real_path(const char *path)
 		if (!is_directory(buf)) {
 			char *last_slash = find_last_dir_sep(buf);
 			if (last_slash) {
-				*last_slash = '\0';
 				last_elem = xstrdup(last_slash + 1);
+				last_slash[1] = '\0';
 			} else {
 				last_elem = xstrdup(buf);
 				*buf = '\0';

t/t0060-path-utils.sh

@@ -148,7 +148,7 @@ test_expect_success 'real path rejects the empty string' '
 	test_must_fail test-path-utils real_path ""
 '
 
-test_expect_failure POSIX 'real path works on absolute paths' '
+test_expect_success POSIX 'real path works on absolute paths' '
 	nopath="hopefully-absent-path" &&
 	test "/" = "$(test-path-utils real_path "/")" &&
 	test "/$nopath" = "$(test-path-utils real_path "/$nopath")" &&