Move CommitExists / Head into internal/vcs/git , drop the stderr-string matching

Move CommitExists / Head into internal/vcs/git , drop the stderr-string matching: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Right now the only implementation of "does this commit exist?" lives on the codeintel gitserver Client in…