is_directory(): a generic helper function

A simple "grep -e stat --and -e S_ISDIR" revealed there are many
open-coded implementations of this function.

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

cache.h

@@ -533,6 +533,7 @@ static inline int is_absolute_path(const char *path)
 {
 	return path[0] == '/' || has_dos_drive_prefix(path);
 }
+int is_directory(const char *);
 const char *make_absolute_path(const char *path);
 const char *make_nonrelative_path(const char *path);
 const char *make_relative_path(const char *abs, const char *base);