builtin/gc.c: make gc.cruftPacks enabled by default

Back in 5b92477f89 (builtin/gc.c: conditionally avoid pruning objects
via loose, 2022-05-20), `git gc` learned the `--cruft` option and
`gc.cruftPacks` configuration to opt-in to writing cruft packs when
collecting or pruning unreachable objects.

Cruft packs were introduced with the merge in a50036da1a (Merge branch
'tb/cruft-packs', 2022-06-03). They address the problem of "loose object
explosions", where Git will write out many individual loose objects when
there is a large number of unreachable objects that have not yet aged
past `--prune=<date>`.

Instead of keeping track of those unreachable yet recent objects via
their loose object file's mtime, cruft packs collect all unreachable
objects into a single pack with a corresponding `*.mtimes` file that
acts as a table to store the mtimes of all unreachable objects. This
prevents the need to store unreachable objects as loose as they age out
of the repository, and avoids the problem of loose object explosions.

Beyond avoiding loose object explosions, cruft packs also act as a more
efficient mechanism to store unreachable objects as they age out of a
repository. This is because pairs of similar unreachable objects serve
as delta bases for one another.

In 5b92477f89, the feature was introduced as experimental. Since then,
GitHub has been running these patches in every repository generating
hundreds of millions of cruft packs along the way. The feature is
battle-tested, and avoids many pathological cases such as above. Users
who either run `git gc` manually, or via `git maintenance` can benefit
from having cruft packs.

As such, enable cruft pack generation to take place by default (by
making `gc.cruftPacks` have the default of "true" rather than "false).

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/feature.txt

@@ -14,9 +14,6 @@ feature.experimental::
 +
 * `fetch.negotiationAlgorithm=skipping` may improve fetch negotiation times by
 skipping more commits at a time, reducing the number of round trips.
-+
-* `gc.cruftPacks=true` reduces disk space used by unreachable objects during
-garbage collection, preventing loose object explosions.
 
 feature.manyFiles::
 	Enable config options that optimize for repos with many files in the

Documentation/config/gc.txt

@@ -84,7 +84,7 @@ gc.packRefs::
 gc.cruftPacks::
 	Store unreachable objects in a cruft pack (see
 	linkgit:git-repack[1]) instead of as loose objects. The default
-	is `false`.
+	is `true`.
 
 gc.pruneExpire::
 	When 'git gc' is run, it will call 'prune --expire 2.weeks.ago'

Documentation/git-gc.txt

@@ -54,9 +54,10 @@ other housekeeping tasks (e.g. rerere, working trees, reflog...) will
 be performed as well.
 
 
---cruft::
+--[no-]cruft::
 	When expiring unreachable objects, pack them separately into a
-	cruft pack instead of storing them as loose objects.
+	cruft pack instead of storing them as loose objects. `--cruft`
+	is on by default.
 
 --prune=<date>::
 	Prune loose objects older than date (default is 2 weeks ago,

Documentation/gitformat-pack.txt

@@ -611,8 +611,8 @@ result of repeatedly resetting the objects' mtimes to the present time.
 
 If you are GC-ing repositories in a mixed version environment, consider omitting
 the `--cruft` option when using linkgit:git-repack[1] and linkgit:git-gc[1], and
-leaving the `gc.cruftPacks` configuration unset until all writers understand
+setting the `gc.cruftPacks` configuration to "false" until all writers
-cruft packs.
+understand cruft packs.
 
 === Alternatives
 

builtin/gc.c

@@ -48,7 +48,7 @@ static const char * const builtin_gc_usage[] = {
 
 static int pack_refs = 1;
 static int prune_reflogs = 1;
-static int cruft_packs = -1;
+static int cruft_packs = 1;
 static int aggressive_depth = 50;
 static int aggressive_window = 250;
 static int gc_auto_threshold = 6700;
@@ -608,10 +608,6 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	if (prune_expire && parse_expiry_date(prune_expire, &dummy))
 		die(_("failed to parse prune expiry value %s"), prune_expire);
 
-	prepare_repo_settings(the_repository);
-	if (cruft_packs < 0)
-		cruft_packs = the_repository->settings.gc_cruft_packs;
-
 	if (aggressive) {
 		strvec_push(&repack, "-f");
 		if (aggressive_depth > 0)

t/t6500-gc.sh

@@ -216,11 +216,9 @@ assert_no_cruft_packs () {
 }
 
 for argv in \
-	"gc --cruft" \
+	"gc" \
 	"-c gc.cruftPacks=true gc" \
-	"-c gc.cruftPacks=false gc --cruft" \
+	"-c gc.cruftPacks=false gc --cruft"
-	"-c feature.experimental=true gc" \
-	"-c gc.cruftPacks=true -c feature.experimental=false gc"
 do
 	test_expect_success "git $argv generates a cruft pack" '
 		test_when_finished "rm -fr repo" &&
@@ -244,11 +242,9 @@ do
 done
 
 for argv in \
-	"gc" \
+	"gc --no-cruft" \
 	"-c gc.cruftPacks=false gc" \
-	"-c gc.cruftPacks=true gc --no-cruft" \
+	"-c gc.cruftPacks=true gc --no-cruft"
-	"-c feature.expiremental=true -c gc.cruftPacks=false gc" \
-	"-c feature.experimental=false gc"
 do
 	test_expect_success "git $argv does not generate a cruft pack" '
 		test_when_finished "rm -fr repo" &&