checkout-index: simplify locking logic

`newfd` starts out negative. If we then take the lock, `newfd` will
become non-negative. We later check for exactly that property before
calling `write_locked_index()`. That is, we are simply using `newfd` as
a boolean to keep track of whether we took the lock or not. (We always
use `newfd` and `lock_file` together, so they really are mirroring each
other.)

Drop `newfd` and check with `is_lock_file_locked()` instead. While at
it, move the `static struct lock_file` into `cmd_checkout_index()` and
make it non-static. It is only used in this function, and after
076aa2cbd (tempfile: auto-allocate tempfiles on heap, 2017-09-05), we
can have lockfiles on the stack.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout-index.c

@@ -129,8 +129,6 @@ static const char * const builtin_checkout_index_usage[] = {
 	NULL
 };
 
-static struct lock_file lock_file;
-
 static int option_parse_stage(const struct option *opt,
 			      const char *arg, int unset)
 {
@@ -150,7 +148,7 @@ static int option_parse_stage(const struct option *opt,
 int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 {
 	int i;
-	int newfd = -1;
+	struct lock_file lock_file = LOCK_INIT;
 	int all = 0;
 	int read_from_stdin = 0;
 	int prefix_length;
@@ -206,7 +204,7 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 	if (index_opt && !state.base_dir_len && !to_tempfile) {
 		state.refresh_cache = 1;
 		state.istate = &the_index;
-		newfd = hold_locked_index(&lock_file, LOCK_DIE_ON_ERROR);
+		hold_locked_index(&lock_file, LOCK_DIE_ON_ERROR);
 	}
 
 	/* Check out named files first */
@@ -251,7 +249,7 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 	if (all)
 		checkout_all(prefix, prefix_length);
 
-	if (0 <= newfd &&
+	if (is_lock_file_locked(&lock_file) &&
 	    write_locked_index(&the_index, &lock_file, COMMIT_LOCK))
 		die("Unable to write new index file");
 	return 0;