"git upload-pack" that serves "git fetch" can be told to serve
commits that are not at the tip of any ref, as long as they are
reachable from a ref, with uploadpack.allowReachableSHA1InWant
configuration variable.
* fm/fetch-raw-sha1:
upload-pack: optionally allow fetching reachable sha1
upload-pack: prepare to extend allow-tip-sha1-in-want
config.txt: clarify allowTipSHA1InWant with camelCase