When opening any files in the object database, release unused pack
windows if the open(2) syscall fails due to EMFILE (too many open
files in this process).  This allows Git to degrade gracefully on
a repository with thousands of pack files, and a commit stored in
a loose object in the middle of the history.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>