Introduce new function real_path_if_valid()

The function is like real_path(), except that it returns NULL on error
instead of dying.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>

cache.h

@@ -714,6 +714,7 @@ static inline int is_absolute_path(const char *path)
 }
 int is_directory(const char *);
 const char *real_path(const char *path);
+const char *real_path_if_valid(const char *path);
 const char *absolute_path(const char *path);
 const char *relative_path(const char *abs, const char *base);
 int normalize_path_copy(char *dst, const char *src);