Summary:
This is a resend of https://phab.mercurial-scm.org/D1430, without breaking
Windows.
I encountered "too many opened files" problem due to treemanifest packs on my
laptop. This patch seems to be the easiest solution without side effects. Other
choices are deleting files (seem like an non-ideal workaround), forcing a
repack (could be slow), and rewriting using Rust (could take too long).
The root cause is Python's `mmap` implementation has to keep a fd internally
to support `mmapobj.resize` API. We only need read-only operation on the
mmap object so the fd is unnecessary. Re-implement a minimal mmap interface
for this purpose.
Reviewed By: DurhamG
Differential Revision: D6835890
fbshipit-source-id: 74c429e957cb8677682604eb02fc38b5b8d13ef7
