url: move drive letter checking into has_drive_letter() for extensions This will let the schemes extension override drive letter detection to allow single letter schemes.

changegroup: unnest some lookup functions

changegroup: minor ordering and renaming changes for similarity This eliminates some needless differences between the two changegroup variants

changegroup: remove nodeiter temporary

changegroup: refactor prune as a filter

changegroup: add first logic to send file header This will allow us to later change how we filter needed file nodes

url: fix tests

url: deal with drive letters