get_relative_cwd(): clarify why it handles dir == NULL

The comment did not make a good case why it makes sense.
Clarify, and remove stale comment about the caller being lazy.
The behaviour on NULL input is pretty much intentional.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -646,16 +646,21 @@ file_exists(const char *f)
 /*
  * get_relative_cwd() gets the prefix of the current working directory
  * relative to 'dir'.  If we are not inside 'dir', it returns NULL.
- * As a convenience, it also returns NULL if 'dir' is already NULL.
+ *
+ * As a convenience, it also returns NULL if 'dir' is already NULL.  The
+ * reason for this behaviour is that it is natural for functions returning
+ * directory names to return NULL to say "this directory does not exist"
+ * or "this directory is invalid".  These cases are usually handled the
+ * same as if the cwd is not inside 'dir' at all, so get_relative_cwd()
+ * returns NULL for both of them.
+ *
+ * Most notably, get_relative_cwd(buffer, size, get_git_work_tree())
+ * unifies the handling of "outside work tree" with "no work tree at all".
  */
 char *get_relative_cwd(char *buffer, int size, const char *dir)
 {
 	char *cwd = buffer;
 
-	/*
-	 * a lazy caller can pass a NULL returned from get_git_work_tree()
-	 * and rely on this function to return NULL.
-	 */
 	if (!dir)
 		return NULL;
 	if (!getcwd(buffer, size))