symbolic link support in commit API
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### Description
I'm using GitLab Enterprise Edition 10.5.4-ee
My application gives users option to perform batch operations on multiple files.
Commit create operation in [Commit API](https://docs.gitlab.com/ee/api/commits.html#create-a-commit-with-multiple-files-and-actions) does not provide an option to create symbolic links.
### Proposal
It should be enough to add `fmode` parameter and allow to set to `120000`.
Or use a separate option to mark file as a symlink.
### Links / references
issue