gc: convert to using the_hash_algo

There's been a lot of changing of the hardcoded "40" values to
the_hash_algo->hexsz, but we've so far missed this one where we
hardcoded 38 for the loose object file length.

This is because a SHA-1 like abcde[...] gets turned into
objects/ab/cde[...]. There's no reason to suppose the same won't be
the case for SHA-256, and reading between the lines in
hash-function-transition.txt the format is planned to be the same.

In the future we may want to further modify this code for the hash
function transition. There's a potential pathological case here where
we'll only consider the loose objects for the currently active hash,
but objects for that hash will share a directory storage with the
other hash.

Thus we could theoretically have e.g. 1k SHA-1 loose objects, and 1
million SHA-256 objects. Then not notice that we need to pack them
because we're currently using SHA-1, even though our FS may be
straining under the stress of such humongous directories.

So assuming that "gc" eventually learns to pack up both SHA-1 and
SHA-256 objects regardless of what the current the_hash_algo is,
perhaps this check should be changed to consider all files in
objects/17/ matching [0-9a-f] 38 or 62 characters in length (i.e. both
SHA-1 and SHA-256).

But none of that is something we need to worry about now, and
supporting both 38 and 62 characters depending on "the_hash_algo"
removes another case of SHA-1 hardcoding.

As noted in [1] I'm making no effort to somehow remove the hardcoding
for "2" as in "use the first two hexdigits for the directory
name". There's no indication that that'll ever change, and somehow
generalizing it here would be a drop in the ocean, so there's no point
in doing that. It also couldn't be done without coming up with some
generalized version of the magical "objects/17" directory. See [2] for
a discussion of that directory.

1. https://public-inbox.org/git/874l84ber7.fsf@evledraar.gmail.com/

2. https://public-inbox.org/git/87k1mta9x5.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/gc.c

@@ -156,6 +156,7 @@ static int too_many_loose_objects(void)
 	int auto_threshold;
 	int num_loose = 0;
 	int needed = 0;
+	const unsigned hexsz_loose = the_hash_algo->hexsz - 2;
 
 	dir = opendir(git_path("objects/17"));
 	if (!dir)
@@ -163,8 +164,8 @@ static int too_many_loose_objects(void)
 
 	auto_threshold = DIV_ROUND_UP(gc_auto_threshold, 256);
 	while ((ent = readdir(dir)) != NULL) {
-		if (strspn(ent->d_name, "0123456789abcdef") != 38 ||
+		if (strspn(ent->d_name, "0123456789abcdef") != hexsz_loose ||
-		    ent->d_name[38] != '\0')
+		    ent->d_name[hexsz_loose] != '\0')
 			continue;
 		if (++num_loose > auto_threshold) {
 			needed = 1;