Implement the rmdir_or_warn function

This patch implements an rmdir_or_warn function (like unlink_or_warn
but for directories) that uses the generalised warning code in
warn_if_unremovable.

Signed-off-by: Peter Collingbourne <peter@pcc.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -469,5 +469,9 @@ void git_qsort(void *base, size_t nmemb, size_t size,
  * Always returns the return value of unlink(2).
  */
 int unlink_or_warn(const char *path);
+/*
+ * Likewise for rmdir(2).
+ */
+int rmdir_or_warn(const char *path);
 
 #endif

wrapper.c

@@ -328,3 +328,8 @@ int unlink_or_warn(const char *file)
 {
 	return warn_if_unremovable("unlink", file, unlink(file));
 }
+
+int rmdir_or_warn(const char *file)
+{
+	return warn_if_unremovable("rmdir", file, rmdir(file));
+}