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: avoid concurrent appends to partial packs http-fetch: correct --index-pack-arg documentation