Merge branch 'tn/packfile-uri-concurrency'

Merge branch 'tn/packfile-uri-concurrency'

Concurrent downloads of packfiles via packfile URIs and dumb HTTP are
safer by avoiding concurrent appends to the staging file.  Opening in
read-write mode with separate file offsets prevents corruption and
preserves resumability.  'fetch-pack' now tolerates pre-existing
'.keep' files.

* tn/packfile-uri-concurrency:
  fetch-pack: accept "pack" output for packfile URIs
  http: permit unlinking partial packs on Windows
  http: avoid concurrent appends to partial packs
  http: accept HTTP 416 for complete partial packs
  http: avoid closing index-pack input twice
  http-fetch: correct --index-pack-arg documentation