maintenance: add incremental-repack auto condition

The incremental-repack task updates the multi-pack-index by deleting pack-
files that have been replaced with new packs, then repacking a batch of
small pack-files into a larger pack-file. This incremental repack is faster
than rewriting all object data, but is slower than some other
maintenance activities.

The 'maintenance.incremental-repack.auto' config option specifies how many
pack-files should exist outside of the multi-pack-index before running
the step. These pack-files could be created by 'git fetch' commands or
by the loose-objects task. The default value is 10.

Setting the option to zero disables the task with the '--auto' option,
and a negative value makes the task run every time.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/gc.c b/builtin/gc.c
index 8d22361..2b99596 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -30,6 +30,7 @@
 #include "promisor-remote.h"
 #include "refs.h"
 #include "remote.h"
+#include "object-store.h"
 
 #define FAILED_RUN "failed to run %s"
 
@@ -1001,6 +1002,35 @@ static int maintenance_task_loose_objects(struct maintenance_run_opts *opts)
 	return prune_packed(opts) || pack_loose(opts);
 }
 
+static int incremental_repack_auto_condition(void)
+{
+	struct packed_git *p;
+	int enabled;
+	int incremental_repack_auto_limit = 10;
+	int count = 0;
+
+	if (git_config_get_bool("core.multiPackIndex", &enabled) ||
+	    !enabled)
+		return 0;
+
+	git_config_get_int("maintenance.incremental-repack.auto",
+			   &incremental_repack_auto_limit);
+
+	if (!incremental_repack_auto_limit)
+		return 0;
+	if (incremental_repack_auto_limit < 0)
+		return 1;
+
+	for (p = get_packed_git(the_repository);
+	     count < incremental_repack_auto_limit && p;
+	     p = p->next) {
+		if (!p->multi_pack_index)
+			count++;
+	}
+
+	return count >= incremental_repack_auto_limit;
+}
+
 static int multi_pack_index_write(struct maintenance_run_opts *opts)
 {
 	struct child_process child = CHILD_PROCESS_INIT;
@@ -1156,6 +1186,7 @@ static struct maintenance_task tasks[] = {
 	[TASK_INCREMENTAL_REPACK] = {
 		"incremental-repack",
 		maintenance_task_incremental_repack,
+		incremental_repack_auto_condition,
 	},
 	[TASK_GC] = {
 		"gc",