Merge branch 'mh/import-transport-fd-fix' into maint

The ownership rule for the file descriptor to fast-import remote
backend was mixed up, leading to unrelated file descriptor getting
closed, which has been fixed.

* mh/import-transport-fd-fix:
Use xmmap_gently instead of xmmap in use_pack
dup() the input fd for fast-import used for remote helpers

diff --cc packfile.c
Simple merge

diff --cc transport-helper.c
Simple merge