bisect: add --force flag to force checkout

This change adds a `--force` flag to `git bisect run` to force the checkout
that happens on each step. Forced checkouts are useful if the command that
`git bisect run` runs modifies the repository state, causing local changes,
such as in package lock files. A developer can run `git bisect run --force`
to ensure that the the repository is reset between each invocation.

bisect.c

@@ -713,7 +713,7 @@ static int is_expected_rev(const struct object_id *oid)
 }
 
 enum bisect_error bisect_checkout(const struct object_id *bisect_rev,
-				  int no_checkout)
+				  int no_checkout, int force_checkout)
 {
 	struct commit *commit;
 	struct pretty_print_context pp = {0};
@@ -728,8 +728,11 @@ enum bisect_error bisect_checkout(const struct object_id *bisect_rev,
 		struct child_process cmd = CHILD_PROCESS_INIT;
 
 		cmd.git_cmd = 1;
-		strvec_pushl(&cmd.args, "checkout", "-q",
-			     oid_to_hex(bisect_rev), "--", NULL);
+		strvec_pushl(&cmd.args, "checkout", "-q");
+		if (force_checkout) {
+			strvec_pushl(&cmd.args, "--force");
+		}
+		strvec_pushl(&cmd.args, oid_to_hex(bisect_rev), "--", NULL);
 		if (run_command(&cmd))
 			/*
 			 * Errors in `run_command()` itself, signaled by res < 0,
@@ -850,7 +853,7 @@ static enum bisect_error check_merge_bases(int rev_nr, struct commit **rev, int
 			handle_skipped_merge_base(mb);
 		} else {
 			printf(_("Bisecting: a merge base must be tested\n"));
-			res = bisect_checkout(mb, no_checkout);
+			res = bisect_checkout(mb, no_checkout, 0);
 			if (!res)
 				/* indicate early success */
 				res = BISECT_INTERNAL_SUCCESS_MERGE_BASE;
@@ -1104,7 +1107,7 @@ enum bisect_error bisect_next_all(struct repository *r, const char *prefix)
 	/* Clean up objects used, as they will be reused. */
 	repo_clear_commit_marks(r, ALL_REV_FLAGS);
 
-	res = bisect_checkout(bisect_rev, no_checkout);
+	res = bisect_checkout(bisect_rev, no_checkout, 0);
 cleanup:
 	release_revisions(&revs);
 	strvec_clear(&rev_argv);

bisect.h

@@ -80,6 +80,6 @@ void read_bisect_terms(const char **bad, const char **good);
 int bisect_clean_state(void);
 
 enum bisect_error bisect_checkout(const struct object_id *bisect_rev,
-				  int no_checkout);
+				  int no_checkout, int force_checkout);
 
 #endif

builtin/bisect.c

@@ -1173,7 +1173,7 @@ static int do_bisect_run(const char *command)
 	return run_command(&cmd);
 }
 
-static int verify_good(const struct bisect_terms *terms, const char *command)
+static int verify_good(const struct bisect_terms *terms, const char *command, int force_checkout)
 {
 	int rc;
 	enum bisect_error res;
@@ -1189,13 +1189,13 @@ static int verify_good(const struct bisect_terms *terms, const char *command)
 	if (read_ref(no_checkout ? "BISECT_HEAD" : "HEAD", &current_rev))
 		return -1;
 
-	res = bisect_checkout(&good_rev, no_checkout);
+	res = bisect_checkout(&good_rev, no_checkout, force_checkout);
 	if (res != BISECT_OK)
 		return -1;
 
 	rc = do_bisect_run(command);
 
-	res = bisect_checkout(&current_rev, no_checkout);
+	res = bisect_checkout(&current_rev, no_checkout, force_checkout);
 	if (res != BISECT_OK)
 		return -1;
 
@@ -1209,6 +1209,7 @@ static int bisect_run(struct bisect_terms *terms, int argc, const char **argv)
 	const char *new_state;
 	int temporary_stdout_fd, saved_stdout;
 	int is_first_run = 1;
+	int force_checkout = 0;
 
 	if (bisect_next_check(terms, NULL))
 		return BISECT_FAILED;
@@ -1218,8 +1219,15 @@ static int bisect_run(struct bisect_terms *terms, int argc, const char **argv)
 		return BISECT_FAILED;
 	}
 
-	sq_quote_argv(&command, argv);
-	strbuf_ltrim(&command);
+	if (argv[0] == "--force") {
+		force_checkout = 1;
+		sq_quote_argv(&command, argv[1]);
+		strbuf_ltrim(&command);
+	} else {
+		sq_quote_argv(&command, argv);
+		strbuf_ltrim(&command);
+	}
+
 	while (1) {
 		res = do_bisect_run(command.buf);
 
@@ -1231,7 +1239,7 @@ static int bisect_run(struct bisect_terms *terms, int argc, const char **argv)
 		 * missing or non-executable script.
 		 */
 		if (is_first_run && (res == 126 || res == 127)) {
-			int rc = verify_good(terms, command.buf);
+			int rc = verify_good(terms, command.buf, force_checkout);
 			is_first_run = 0;
 			if (rc < 0 || 128 <= rc) {
 				error(_("unable to verify %s on good"