- Apply some refactoring for code reuse
- Add file status validation for Files::DeleteService
- Write additional specs

When commiting multiple files we're now checking if any of those files
has been modified by another commit and we're rejecting the new commit
in this case.

Closes #35692.

Documentation bugfix of invalid JSON payload example of Create a commit with multiple files and actions

Fixes #23895.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

add docs and tests - add additional validation
allow move without content
updated response

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The attribute to filter by branch or tag needs to be named `ref`,
not `ref_name`. And indeed the attribute in the JSON response is `ref`
(and not `ref_name`).

Tested on Gitlab CE 8.9.

- Parameter validation as ISO8601 format

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Closes #3196