Merge branch 'km/empty-repo-is-still-a-repo'

Running "git add" on a repository created inside the current
repository is an explicit indication that the user wants to add it
as a submodule, but when the HEAD of the inner repository is on an
unborn branch, it cannot be added as a submodule.  Worse, the files
in its working tree can be added as if they are a part of the outer
repository, which is not what the user wants.  These problems are
being addressed.

* km/empty-repo-is-still-a-repo:
  add: error appropriately on repository with no commits
  dir: do not traverse repositories with no commits
  submodule: refuse to add repository with no commits

git-submodule.sh

@@ -232,6 +232,13 @@ cmd_add()
 		die "$(eval_gettext "'\$sm_path' already exists in the index and is not a submodule")"
 	fi
 
+	if test -d "$sm_path" &&
+		test -z $(git -C "$sm_path" rev-parse --show-cdup 2>/dev/null)
+	then
+	    git -C "$sm_path" rev-parse --verify -q HEAD >/dev/null ||
+	    die "$(eval_gettext "'\$sm_path' does not have a commit checked out")"
+	fi
+
 	if test -z "$force" &&
 		! git add --dry-run --ignore-missing --no-warn-embedded-repo "$sm_path" > /dev/null 2>&1
 	then