maintner: change representation of gitHash to string, intern them

Saves a tiny amount of memory, but many fewer allocation.

name   old time/op    new time/op    delta
Get-4     4.97s ±19%     5.22s ± 9%    ~     (p=0.421 n=5+5)

name   old alloc/op   new alloc/op   delta
Get-4    1.10GB ± 0%    1.09GB ± 0%  -0.67%  (p=0.008 n=5+5)

name   old allocs/op  new allocs/op  delta
Get-4     14.2M ± 0%     12.9M ± 0%  -9.38%  (p=0.008 n=5+5)

Change-Id: Ia4d383b7115a856de517002357c069e6b3eee7ef
Reviewed-on: https://go-review.googlesource.com/38782
Reviewed-by: Kevin Burke <kev@inburke.com>
3 files changed