better prevention of auto repack

Looking through the git sources (documentation is unclear),
it seems commit doesn't ever trigger git-gc, mostly fetching and merging
seems to. I cannot easily override the setting in all those places, so
instead set gc.auto in git config when initializing a repository with
the assistant.

This does mean that the user cannot set gc.auto=0 and completely avoid
repacks, as the assistant does it daily. But, it only does it after there
are 100x the default number of loose objects, so this is probably not going
to be too annoying.

debian/changelog

@@ -9,9 +9,10 @@ git-annex (4.20130228) UNRELEASED; urgency=low
     files in direct mode repositories.
   * assistant: Fix dropping content when a file is moved to an archive
     directory.
-  * assistant: Prevent automatic commits from causing git-gc runs,
-    as that can make things quite slow. Instead, git-gc --auto is run
-    once a day. (This can be disabled by the usual gc.auto=0 setting.)
+  * assistant: Set gc.auto=0 when creating repositories to prevent
+    automatic commits from causing git-gc runs.
+  * assistant: If gc.auto=0, run git-gc once a day, packing loose objects
+    very non-aggressively.
 
  -- Joey Hess <joeyh@debian.org>  Wed, 27 Feb 2013 23:20:40 -0400