lib: add aur-fetch--mirror
lib: add aur-fetch--mirror

Helper script to do a shallow clone of the new AUR mirror [1] and
checkout branches one-by-one. Similar to asp(1), this offers the option
to keep local clones for each branch. This is transparent to aur-fetch
and aur-view in particular.

[1] https://github.com/archlinux/aur

AladW/aurutils,AladW/aurutils