Merge branch 'tn/packfile-uri-concurrency' into seen

Merge branch 'tn/packfile-uri-concurrency' into seen

Concurrent downloads of packfiles via packfile URIs and dumb HTTP have
been made safer by avoiding concurrent appends to the staging file.
Opening the file in read-write mode and maintaining separate file
offsets prevents corruption while preserving resumability.  The
'fetch-pack' command has also been updated to tolerate 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