[PATCH] get_relative_cwd(): clarify why it handles dir == NULL

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

The comment did not make a good case why it makes sense.  It does so now.

Pointed out by Junio Hamano.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 dir.c |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/dir.c b/dir.c
index b3329f4..791c566 100644
--- a/dir.c
+++ b/dir.c
@@ -646,7 +646,16 @@ 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)
 {
-- 
1.5.3.rc3.112.gf60b6