Merge pull request git-for-windows#34 Add Trace2 regions to 'pack-objects'

We want to make `git push` faster, but we need to know where the time is going!

There are likely four places where the time is going:

1. The info/refs call and force-update checking at the beginning.
2. The `git pack-objects` call that creates a pack-file to send to the server.
3. Sending the data to the server.
4. Waiting for the server to verify the pack-file.

This PR adds `trace2_region_` calls inside `git pack-objects` so we can track the time in item (2). The rest could be interpreted from the start and end time of the entire command after we know this region. The server-side verification is something we can track using server telemetry.

Author: derrickstolee

builtin/pack-objects.c

@@ -31,6 +31,7 @@
 #include "packfile.h"
 #include "object-store.h"
 #include "dir.h"
+#include "trace2.h"
 
 #define IN_PACK(obj) oe_in_pack(&to_pack, obj)
 #define SIZE(obj) oe_size(&to_pack, obj)
@@ -3356,6 +3357,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	trace2_region_enter("pack-objects", "enumerate-objects", the_repository);
 	prepare_packing_data(&to_pack);
 
 	if (progress)
@@ -3370,12 +3372,20 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	if (include_tag && nr_result)
 		for_each_ref(add_ref_tag, NULL);
 	stop_progress(&progress_state);
+	trace2_region_leave("pack-objects", "enumerate-objects", the_repository);
 
 	if (non_empty && !nr_result)
 		return 0;
-	if (nr_result)
+	if (nr_result) {
+		trace2_region_enter("pack-objects", "prepare-pack", the_repository);
 		prepare_pack(window, depth);
+		trace2_region_leave("pack-objects", "prepare-pack", the_repository);
+	}
+
+	trace2_region_enter("pack-objects", "write-pack-file", the_repository);
 	write_pack_file();
+	trace2_region_leave("pack-objects", "write-pack-file", the_repository);
+
 	if (progress)
 		fprintf_ln(stderr,
 			   _("Total %"PRIu32" (delta %"PRIu32"),"