safe_create_leading_directories(): on Windows, \ can separate path components

When cloning to a directory "C:\foo\bar" from Windows' cmd.exe where
"foo" does not exist yet, Git would throw an error like

    fatal: could not create work tree dir 'c:\foo\bar'.: No such file or directory

Fix this by not hard-coding a platform specific directory separator
into safe_create_leading_directories().

This patch, including its entire commit message, is derived from a
patch by Sebastian Schuberth.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -112,17 +112,21 @@ enum scld_error safe_create_leading_directories(char *path)
 
 	while (ret == SCLD_OK && next_component) {
 		struct stat st;
-		char *slash = strchr(next_component, '/');
+		char *slash = next_component, slash_character;
 
-		if (!slash)
+		while (*slash && !is_dir_sep(*slash))
+			slash++;
+
+		if (!*slash)
 			break;
 
 		next_component = slash + 1;
-		while (*next_component == '/')
+		while (is_dir_sep(*next_component))
 			next_component++;
 		if (!*next_component)
 			break;
 
+		slash_character = *slash;
 		*slash = '\0';
 		if (!stat(path, &st)) {
 			/* path exists */
@@ -148,7 +152,7 @@ enum scld_error safe_create_leading_directories(char *path)
 		} else if (adjust_shared_perm(path)) {
 			ret = SCLD_PERMS;
 		}
-		*slash = '/';
+		*slash = slash_character;
 	}
 	return ret;
 }