submodule: absorb git dir instead of dying on deinit

Currently, running 'git submodule deinit' on repos where the
submodule's '.git' is a folder aborts with a message that is not
exactly user friendly. Let's change this to instead warn the user
to rerun the command with '--force'.

This internally calls 'absorb_git_dir_into_superproject()', which
moves the '.git' folder into the superproject and replaces it with
a '.git' file. The rest of the deinit function can operate as it
already does with new-style submodules.

We also edit a test case such that it matches the new behaviour of
deinit.

Suggested-by: Atharva Raykar <raykar.ath@gmail.com>
Signed-off-by: Mugdha Pattnaik <mugdhapattnaik@gmail.com>

builtin/submodule--helper.c

@@ -1539,16 +1539,24 @@ 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)"),
-			    displaypath);
+		if (is_directory(sub_git_dir)) {
+			if (!(flags & OPT_FORCE))
+				die(_("Submodule work tree '%s' contains a "
+					  ".git directory.\nUse --force if you want "
+					  "to move its contents to superproject's "
+					  "module folder and convert .git to a file "
+					  "and then proceed with deinit."),
+					displaypath);
+
+			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(displaypath, flags);
+
+		}
 
 		if (!(flags & OPT_FORCE)) {
 			struct child_process cp_rm = CHILD_PROCESS_INIT;

t/t7400-submodule-basic.sh

@@ -1182,18 +1182,18 @@ 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 fails when submodule has a .git directory unless forced' '
 	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 "$(git config --get-regexp "submodule\.example\.")"
+	git submodule deinit -f init &&
+	! test -d init/.git &&
+	test -z "$(git config --get-regexp "submodule\.example\.")"
 '
 
 test_expect_success 'submodule with UTF-8 name' '