gc: add --keep-largest-pack option

This adds a new repack mode that combines everything into a secondary
pack, leaving the largest pack alone.

This could help reduce memory pressure. On linux-2.6.git, valgrind
massif reports 1.6GB heap in "pack all" case, and 535MB in "pack
all except the base pack" case. We save roughly 1GB memory by
excluding the base pack.

This should also lower I/O because we don't have to rewrite a giant
pack every time (e.g. for linux-2.6.git that's a 1.4GB pack file)..

PS. The use of string_list here seems overkill, but we'll need it in
the next patch...

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-gc.txt

@@ -9,7 +9,7 @@ git-gc - Cleanup unnecessary files and optimize the local repository
 SYNOPSIS
 --------
 [verse]
-'git gc' [--aggressive] [--auto] [--quiet] [--prune=<date> | --no-prune] [--force]
+'git gc' [--aggressive] [--auto] [--quiet] [--prune=<date> | --no-prune] [--force] [--keep-largest-pack]
 
 DESCRIPTION
 -----------
@@ -84,6 +84,10 @@ be performed as well.
 	Force `git gc` to run even if there may be another `git gc`
 	instance running on this repository.
 
+--keep-largest-pack::
+	All packs except the largest pack and those marked with a
+	`.keep` files are consolidated into a single pack.
+
 Configuration
 -------------