nix-prefetch-git: whitespace cleanup

Use 4 spaces per indent level instead of a mix of 2 spaces, 4 spaces and
hardtab. (According to nixpkgs coding style for shell scripts.)
youtube-dl: update from 2014.10.25 to 2014.10.30
nasm: update from 2.11.05 to 2.11.06
libgit2: update from 0.21.1 to 0.21.2
pwgen: update from 2.06 to 2.07
libmicrohttpd: update from 0.9.37 to 0.9.38