gc --auto: raise default auto pack limit from 20 to 50

Recent discussion on the list, with the improvement f7c22cc (always start
looking up objects in the last used pack first, 2007-05-30) brought in,
reached the concensus that the current default 20 is too low.

Reference: http://thread.gmane.org/gmane.comp.version-control.git/77586
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -582,7 +582,7 @@ gc.autopacklimit::
 	When there are more than this many packs that are not
 	marked with `*.keep` file in the repository, `git gc
 	--auto` consolidates them into one larger pack.  The
-	default	value is 20.  Setting this to 0 disables it.
+	default	value is 50.  Setting this to 0 disables it.
 
 gc.packrefs::
 	`git gc` does not run `git pack-refs` in a bare repository by

builtin-gc.c

@@ -25,7 +25,7 @@ static const char * const builtin_gc_usage[] = {
 static int pack_refs = 1;
 static int aggressive_window = -1;
 static int gc_auto_threshold = 6700;
-static int gc_auto_pack_limit = 20;
+static int gc_auto_pack_limit = 50;
 static char *prune_expire = "2.weeks.ago";
 
 #define MAX_ADD 10