repack: introduce new compilation unit

Over the years, builtin/repack.c has turned into a grab-bag of
functionality powering the 'git repack' builtin. Among its many
capabilities, it:

 - can build and spawn 'git pack-objects' commands, which in turn
   generate new packs
 - has infrastructure to manage the set of existing packs in a
   repository
 - has infrastructure to split a sequence of packs into a geometric
   progression based on object size
 - can manage both generating and combining cruft packs together
 - can write new MIDXs

to name a few.

As a result, this builtin has accumulated a lot of code, making adding
new functionality difficult. In the future, 'repack' will learn how to
manage a chain of incremental MIDXs, adding yet more functionality into
the builtin.

As a prerequisite step, let's first move some of the functionality in
the builtin into its own repack.[ch].

This will be done over the course of many steps, since there are many
individual components, some of which will end up in other, yet-to-exist
compilation units of their own. Some of the code movement here is also
non-trivial, so performing it in individual steps will make it easier to
verify.

Let's start by migrating 'struct pack_objects_args' (and the related
corresponding pack_objects_args_release() function) into repack.h, and
teach both the Makefile and Meson how to build the new compilation unit.

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

Author: ttaylorr

Makefile

@@ -1136,6 +1136,7 @@ LIB_OBJS += refs/packed-backend.o
 LIB_OBJS += refs/ref-cache.o
 LIB_OBJS += refspec.o
 LIB_OBJS += remote.o
+LIB_OBJS += repack.o
 LIB_OBJS += replace-object.o
 LIB_OBJS += repo-settings.o
 LIB_OBJS += repository.o

builtin/repack.c

@@ -19,6 +19,7 @@
 #include "prune-packed.h"
 #include "odb.h"
 #include "promisor-remote.h"
+#include "repack.h"
 #include "shallow.h"
 #include "pack.h"
 #include "pack-bitmap.h"
@@ -53,21 +54,6 @@ static const char incremental_bitmap_conflict_error[] = N_(
 "--no-write-bitmap-index or disable the pack.writeBitmaps configuration."
 );
 
-struct pack_objects_args {
-	char *window;
-	char *window_memory;
-	char *depth;
-	char *threads;
-	unsigned long max_pack_size;
-	int no_reuse_delta;
-	int no_reuse_object;
-	int quiet;
-	int local;
-	int name_hash_version;
-	int path_walk;
-	struct list_objects_filter_options filter_options;
-};
-
 static int repack_config(const char *var, const char *value,
 			 const struct config_context *ctx, void *cb)
 {
@@ -116,15 +102,6 @@ static int repack_config(const char *var, const char *value,
 	return git_default_config(var, value, ctx, cb);
 }
 
-static void pack_objects_args_release(struct pack_objects_args *args)
-{
-	free(args->window);
-	free(args->window_memory);
-	free(args->depth);
-	free(args->threads);
-	list_objects_filter_release(&args->filter_options);
-}
-
 struct existing_packs {
 	struct repository *repo;
 	struct string_list kept_packs;

meson.build

@@ -462,6 +462,7 @@ libgit_sources = [
   'reftable/tree.c',
   'reftable/writer.c',
   'remote.c',
+  'repack.c',
   'replace-object.c',
   'repo-settings.c',
   'repository.c',

repack.c

@@ -0,0 +1,11 @@
+#include "git-compat-util.h"
+#include "repack.h"
+
+void pack_objects_args_release(struct pack_objects_args *args)
+{
+	free(args->window);
+	free(args->window_memory);
+	free(args->depth);
+	free(args->threads);
+	list_objects_filter_release(&args->filter_options);
+}

repack.h

@@ -0,0 +1,23 @@
+#ifndef REPACK_H
+#define REPACK_H
+
+#include "list-objects-filter-options.h"
+
+struct pack_objects_args {
+	char *window;
+	char *window_memory;
+	char *depth;
+	char *threads;
+	unsigned long max_pack_size;
+	int no_reuse_delta;
+	int no_reuse_object;
+	int quiet;
+	int local;
+	int name_hash_version;
+	int path_walk;
+	struct list_objects_filter_options filter_options;
+};
+
+void pack_objects_args_release(struct pack_objects_args *args);
+
+#endif /* REPACK_H */