real_path: do not assume '/' is the path seperator

real_path currently assumes it's input had '/' as path seperator.
This assumption does not hold true for the code-path from
prefix_path (on Windows), where real_path can be called before
normalize_path_copy.

Fix real_path so it doesn't make this assumption. Create a helper
function to reverse-search for the last path-seperator in a string.

Signed-off-by: Theo Niessink <theo@taletn.com>
Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -215,6 +215,10 @@ extern char *gitbasename(char *);
 #define is_dir_sep(c) ((c) == '/')
 #endif
 
+#ifndef find_last_dir_sep
+#define find_last_dir_sep(path) strrchr(path, '/')
+#endif
+
 #if __HP_cc >= 61000
 #define NORETURN __attribute__((noreturn))
 #define NORETURN_PTR