submodule: absorb git dir instead of dying on deinit

Currently, running 'git submodule deinit' on repos where the
submodule's '.git' is a directory, aborts with a message that is not
exactly user friendly.

Let's change this to instead warn the user that the .git/ directory
has been absorbed into the superproject.
The rest of the deinit function can operate as it already does with
new-style submodules.

In one test, we used to require "git submodule deinit" to fail even
with the "--force" option when the submodule's .git/ directory is not
absorbed. Adjust it to expect the operation to pass.

Suggested-by: Atharva Raykar <raykar.ath@gmail.com>
Signed-off-by: Mugdha Pattnaik <mugdhapattnaik@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1503,17 +1503,18 @@ static void deinit_submodule(const char *path, const char *prefix,
 		struct strbuf sb_rm = STRBUF_INIT;
 		const char *format;
 
-		/*
-		 * protect submodules containing a .git directory
-		 * NEEDSWORK: instead of dying, automatically call
-		 * absorbgitdirs and (possibly) warn.
-		 */
-		if (is_directory(sub_git_dir))
-			die(_("Submodule work tree '%s' contains a .git "
-			      "directory (use 'rm -rf' if you really want "
-			      "to remove it including all of its history)"),
+		if (is_directory(sub_git_dir)) {
+			if (!(flags & OPT_QUIET))
+				warning(_("Submodule work tree '%s' contains a .git "
+					  "directory. This will be replaced with a "
+					  ".git file by using absorbgitdirs."),
 					displaypath);
 
+			absorb_git_dir_into_superproject(path,
+							 ABSORB_GITDIR_RECURSE_SUBMODULES);
+
+		}
+
 		if (!(flags & OPT_FORCE)) {
 			struct child_process cp_rm = CHILD_PROCESS_INIT;
 			cp_rm.git_cmd = 1;

t/t7400-submodule-basic.sh

@@ -1182,18 +1182,17 @@ test_expect_success 'submodule deinit is silent when used on an uninitialized su
 	rmdir init example2
 '
 
-test_expect_success 'submodule deinit fails when submodule has a .git directory even when forced' '
+test_expect_success 'submodule deinit absorbs .git directory if .git is a directory' '
 	git submodule update --init &&
 	(
 		cd init &&
 		rm .git &&
-		cp -R ../.git/modules/example .git &&
+		mv ../.git/modules/example .git &&
 		GIT_WORK_TREE=. git config --unset core.worktree
 	) &&
-	test_must_fail git submodule deinit init &&
-	test_must_fail git submodule deinit -f init &&
-	test -d init/.git &&
-	test -n "$(git config --get-regexp "submodule\.example\.")"
+	git submodule deinit init &&
+	test_path_is_missing init/.git &&
+	test -z "$(git config --get-regexp "submodule\.example\.")"
 '
 
 test_expect_success 'submodule with UTF-8 name' '