reset: improve worktree safety valves

The existing code checked to make sure we were not in a bare
repository when doing a hard reset. However, we should take
this one step further, and make sure we are in a worktree.
Otherwise, we can end up munging files inside of '.git'.

Furthermore, we should do the same check for --merge resets,
which have the same properties. Actually, a merge reset of
HEAD^ would already complain, since further down in the code
we want a worktree. However, it is nicer to check up-front;
then we are sure we cover all cases ("git reset --merge"
would run, even though it wasn't doing anything) and we can
give a more specific message.

Add tests to t7103 to cover these cases and some missing ones.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-reset.c

@@ -286,8 +286,10 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (reset_type == NONE)
 		reset_type = MIXED; /* by default */
 
-	if (reset_type == HARD && is_bare_repository())
-		die("hard reset makes no sense in a bare repository");
+	if ((reset_type == HARD || reset_type == MERGE)
+	    && !is_inside_work_tree())
+		die("%s reset requires a work tree",
+		    reset_type_names[reset_type]);
 
 	/* Soft reset does not touch the index file nor the working tree
 	 * at all, but requires them in a good order.  Other resets reset

t/t7103-reset-bare.sh

@@ -11,16 +11,42 @@ test_expect_success 'setup non-bare' '
 	git commit -a -m two
 '
 
+test_expect_success 'hard reset requires a worktree' '
+	(cd .git &&
+	 test_must_fail git reset --hard)
+'
+
+test_expect_success 'merge reset requires a worktree' '
+	(cd .git &&
+	 test_must_fail git reset --merge)
+'
+
+test_expect_success 'mixed reset is ok' '
+	(cd .git && git reset)
+'
+
+test_expect_success 'soft reset is ok' '
+	(cd .git && git reset --soft)
+'
+
 test_expect_success 'setup bare' '
 	git clone --bare . bare.git &&
 	cd bare.git
 '
 
-test_expect_success 'hard reset is not allowed' '
-	test_must_fail  git reset --hard HEAD^
+test_expect_success 'hard reset is not allowed in bare' '
+	test_must_fail git reset --hard HEAD^
+'
+
+test_expect_success 'merge reset is not allowed in bare' '
+	test_must_fail git reset --merge HEAD^
+'
+
+test_expect_success 'mixed reset is not allowed in bare' '
+	test_must_fail git reset --mixed HEAD^
 '
 
-test_expect_success 'soft reset is allowed' '
+test_expect_success 'soft reset is allowed in bare' '
 	git reset --soft HEAD^ &&
 	test "`git show --pretty=format:%s | head -n 1`" = "one"
 '