pack-objects: create pack.useSparse setting

The '--sparse' flag in 'git pack-objects' changes the algorithm
used to enumerate objects to one that is faster for individual
users pushing new objects that change only a small cone of the
working directory. The sparse algorithm is not recommended for a
server, which likely sends new objects that appear across the
entire working directory.

Create a 'pack.useSparse' setting that enables this new algorithm.
This allows 'git push' to use this algorithm without passing a
'--sparse' flag all the way through four levels of run_command()
calls.

If the '--no-sparse' flag is set, then this config setting is
overridden.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/pack.txt

@@ -105,6 +105,15 @@ pack.useBitmaps::
 	true. You should not generally need to turn this off unless
 	you are debugging pack bitmaps.
 
+pack.useSparse::
+	When true, git will default to using the '--sparse' option in
+	'git pack-objects' when the '--revs' option is present. This
+	algorithm only walks trees that appear in paths that introduce new
+	objects. This can have significant performance benefits when
+	computing a pack to send a small change. However, it is possible
+	that extra objects are added to the pack-file if the included
+	commits contain certain types of direct renames.
+
 pack.writeBitmaps (deprecated)::
 	This is a deprecated synonym for `repack.writeBitmaps`.
 

builtin/pack-objects.c

@@ -2711,6 +2711,10 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 		use_bitmap_index_default = git_config_bool(k, v);
 		return 0;
 	}
+	if (!strcmp(k, "pack.usesparse")) {
+		sparse = git_config_bool(k, v);
+		return 0;
+	}
 	if (!strcmp(k, "pack.threads")) {
 		delta_search_threads = git_config_int(k, v);
 		if (delta_search_threads < 0)

t/t5322-pack-objects-sparse.sh

@@ -118,4 +118,19 @@ test_expect_success 'sparse pack-objects' '
 	test_cmp expect_sparse_objects.txt sparse_objects.txt
 '
 
+test_expect_success 'pack.useSparse enables algorithm' '
+	git config pack.useSparse true &&
+	git pack-objects --stdout --revs <packinput.txt >sparse.pack &&
+	git index-pack -o sparse.idx sparse.pack &&
+	git show-index <sparse.idx | awk "{print \$2}" >sparse_objects.txt &&
+	test_cmp expect_sparse_objects.txt sparse_objects.txt
+'
+
+test_expect_success 'pack.useSparse overridden' '
+	git pack-objects --stdout --revs --no-sparse <packinput.txt >sparse.pack &&
+	git index-pack -o sparse.idx sparse.pack &&
+	git show-index <sparse.idx | awk "{print \$2}" >sparse_objects.txt &&
+	test_cmp required_objects.txt sparse_objects.txt
+'
+
 test_done