nix-prefetch-git: Convert relative submodule URLS to absolute URLS

nix-prefetch-git does not convert relative submodule urls into absolute
urls based on the parent's origin. This patch adds support for
repositories which are using the relative url syntax.
darcsver: Upgrade from 1.7.2 -> 1.7.4
pycryptopp: Upgrade from 0.5.29 ->
0.6.0.1206569328141510525648634803928199668821045408958
twisted: Upgrade from 10.2.0 -> 13.2.0
zfec: Upgrade from 1.4.7 -> 1.4.24
tahoe-lafs: Upgrade from 1.9.2 -> 1.10.0