Have a way to make depend MR from other MR

Request by @jadahl:

Personally I'd like to see some way for merge requests to depend on each other. For example, if development of various features depend on a common functionality, but the features end up ready for review before the common functionality merge request was landed, the merge requests for the features will all contain the commits of the common feature.

For a more explicit example, consider we have the MR (a) containing

abc123fe Some needed refactoring
45ef1344 Add some important infrastructure

that is awaiting review. We then have two separate features building on top of it:

a98beff3 Implement support for this and that

and

123edcba Add fancy new thingy

There is currently no way to create merge requests for the second two, that doesn't also include the commits of MR (a).

Previously in Bugzilla, this was dealt with by simply just attaching the patches that one expected to have reviewed, while stating that they depend on the patches in another bug.

Upstream feature request issue of something that sounds like it should be able to handle this: https://gitlab.com/gitlab-org/gitlab-ce/issues/14232

Upstream link - https://gitlab.com/gitlab-org/gitlab-ce/issues/14232