When passing start_branch on committing from the WebIDE, it's possible
that the branch has changed since editing started, which results in the
change being applied on top of the latest commit in the branch and
overwriting the new changes.

By passing the start_sha instead we can make sure that the change is
applied on top of the commit which the user started editing from.

This reverts commit ec4730478b798270781257913ee4cede673d4d4e.

This adds an endpoint to get the common ancestor of 2 refs from the API.

Signed-off-by: NRémy Coutable <remy@rymai.me>

- Add tests for Repository#write_config

Having a distinct class just for that was a bit overkill

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7237#note_19747856

DRY code + fix rubocop

Add more test cases

Append to changelog

DRY changes list

find_url service for merge_requests

use GET for getting merge request links

remove files

rename to get_url_service

reduce loop

add test case for cross project

refactor tiny thing

update changelog

Also remove the spec for it

commit starts with 7 zeros.