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…

The task

Right now the only implementation of "does this commit exist?" lives on the codeintel gitserver `Client` in `enterprise/internal/codeintel/gitserver/client.go`. The way it decides whether a commit exists is:

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.