You might end up with a situation where you have tons of pack files, e.g.
when using hg2git.  In this situation, all kinds of operations may
end up with a "too many files open" error.  Let's recover gracefully from
that.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Looks-right-to-me-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>