config: warn on inaccessible files

Before reading a config file, we check "!access(path, R_OK)"
to make sure that the file exists and is readable. If it's
not, then we silently ignore it.

For the case of ENOENT, this is fine, as the presence of the
file is optional. For other cases, though, it may indicate a
configuration error (e.g., not having permissions to read
the file). Let's print a warning in these cases to let the
user know.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -604,6 +604,9 @@ int rmdir_or_warn(const char *path);
  */
 int remove_or_warn(unsigned int mode, const char *path);
 
+/* Call access(2), but warn for any error besides ENOENT. */
+int access_or_warn(const char *path, int mode);
+
 /* Get the passwd entry for the UID of the current process. */
 struct passwd *xgetpwuid_self(void);