Add support for "last_commit_id" in commit create api

TLDR; same as gitlab-ce#32642, but for commit api

Description

I'm using GitLab CE 9.3 REST API v4 to perform file read and update operations.

My application gives users option to perform batch operations on multiple files.

Commit create operation in Commit API does not provide an option to detect concurrent file modification (i.e. some other user updated same file during my edit session).

Proposal

Introduce optimistic locking support via optional parameter last_commit_id on each file affected by new commit (at least for update/delete).

I'm ready to do MR for this feature, if there is no workaround.