unpack-trees: add special cwd handling

When running commands such as `git reset --hard` from a subdirectory, if
that subdirectory is in the way of adding needed files, bail with an
error message.

Note that this change looks kind of like it duplicates the new lines of
code from the previous commit in verify_clean_subdirectory().  However,
when we are preserving untracked files, we would rather any error
messages about untracked files being in the way take precedence over
error messages about a subdirectory that happens to be the_original_cwd
being in the way.  But in the UNPACK_RESET_OVERWRITE_UNTRACKED case,
there is no untracked checking to be done, so we simply add a special
case near the top of verify_absent_1.

Acked-by: Derrick Stolee <stolee@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2501-cwd-empty.sh

@@ -121,7 +121,7 @@ test_expect_success 'reset --hard does not clean cwd incidentally' '
 '
 
 test_expect_success 'reset --hard fails if cwd needs to be removed' '
-	test_required_dir_removal failure git reset --hard fd_conflict
+	test_required_dir_removal success git reset --hard fd_conflict
 '
 
 test_expect_success 'merge does not clean cwd incidentally' '

unpack-trees.c

@@ -2261,10 +2261,19 @@ static int verify_absent_1(const struct cache_entry *ce,
 	int len;
 	struct stat st;
 
-	if (o->index_only || !o->update ||
-	    o->reset == UNPACK_RESET_OVERWRITE_UNTRACKED)
+	if (o->index_only || !o->update)
 		return 0;
 
+	if (o->reset == UNPACK_RESET_OVERWRITE_UNTRACKED) {
+		/* Avoid nuking startup_info->original_cwd... */
+		if (startup_info->original_cwd &&
+		    !strcmp(startup_info->original_cwd, ce->name))
+			return add_rejected_path(o, ERROR_CWD_IN_THE_WAY,
+						 ce->name);
+		/* ...but nuke anything else. */
+		return 0;
+	}
+
 	len = check_leading_path(ce->name, ce_namelen(ce), 0);
 	if (!len)
 		return 0;