Enables cloning and fetching from bundles.

I implemented it as a new "transport", so the entry point is in transport_find_fn, where we check if a local file is a bundle (but only after we have attempted to find other transport options).

The documentation I used to implement this is here:

Git: Bundle File Format Documentation
Git: Bundle Documentation

Still Todo:

• Enforce the prerequisite commits when present. I think this would be done in bundle_negotiate_fetch.
• Add a libgit2 API to create bundles.
• Show progress reading the pack?

Some puzzles:

• I had to add "sort_head_to_front" as clients of ls_remote such as local clone require HEAD to be the first in the list.
• read_until_packfile felt a bit dirty/inefficient reading byte-by-byte until the double newline, which denotes the start of the packfile.

Closes#6824 and#1718.