lock_file: make function-local locks non-static

Placing `struct lock_file`s on the stack used to be a bad idea, because
the temp- and lockfile-machinery would keep a pointer into the struct.
But after 076aa2cbd (tempfile: auto-allocate tempfiles on heap,
2017-09-05), we can safely have lockfiles on the stack. (This applies
even if a user returns early, leaving a locked lock behind.)

These `struct lock_file`s are local to their respective functions and we
can drop their staticness.

For good measure, I have inspected these sites and come to believe that
they always release the lock, with the possible exception of bailing out
using `die()` or `exit()` or by returning from a `cmd_foo()`.

As pointed out by Jeff King, it would be bad if someone held on to a
`struct lock_file *` for some reason. After some grepping, I agree with
his findings: no-one appears to be doing that.

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

bundle.c

@@ -409,7 +409,7 @@ static int write_bundle_refs(int bundle_fd, struct rev_info *revs)
 int create_bundle(struct bundle_header *header, const char *path,
 		  int argc, const char **argv)
 {
-	static struct lock_file lock;
+	struct lock_file lock = LOCK_INIT;
 	int bundle_fd = -1;
 	int bundle_to_stdout;
 	int ref_count = 0;