dev/sg: use TrimPrefix instead of TrimLeft

Created by: bobheadxi

TrimLeft removes all characters in the cutset, so:

strings.TrimLeft("dev-e3b35a01fae596c6ca75610081cd38212de2bb68", "dev-")
// "3b35a01fae596c6ca75610081cd38212de2bb68"

this is because the leading e in the commit is trimmed out because e is in dev-. what I actually want here is TrimPrefix

Updates another usage of TrimLeft that was likely intended to be TrimPrefix as well

Test plan

n/a, hard to generate a commit that starts with d, e, or v but this should work