Merge branch 'bc/unuse-packfile'

Handle memory pressure and file descriptor pressure separately when
deciding to release pack windows to honor resource limits.

* bc/unuse-packfile:
Don't close pack fd when free'ing pack windows
sha1_file: introduce close_one_pack() to close packs on fd pressure

diff --cc git-compat-util.h
Simple merge

diff --cc sha1_file.c
Simple merge