Oid: use Go's conversion functions
Oid: use Go's conversion functions

Go already has all the necessary pieces for encoding and decoding hex
strings. Using them let's us avoid going into C land.

Benchmarks show this takes about half the time as using libgit2's
functions.

joseferminj/git2go,ezwiebel/git2go,clearr/git2go,clns/git2go,hwjeong/git2go,clearr/git2go,TheDahv/git2go,treejames/git2go,Acidburn0zzz/git2go,kissthink/git2go,orivej/git2go,pks-t/git2go,joseferminj/git2go,clns/git2go,AaronO/git2go,libgit2/git2go,sygool/git2go,hwjeong/git2go,orivej/git2go,treejames/git2go,libgit2/git2go,kissthink/git2go,tiborvass/git2go,hwjeong/git2go,libgit2/git2go,orivej/git2go,tiborvass/git2go,sygool/git2go,TheDahv/git2go,michaeledgar/git2go,ezwiebel/git2go,treejames/git2go,michaeledgar/git2go,TheDahv/git2go,clns/git2go,ezwiebel/git2go,tiborvass/git2go,clearr/git2go,pks-t/git2go,AaronO/git2go,Acidburn0zzz/git2go,joseferminj/git2go