Support backport branch targets when merging

The merge action should gain knowledge of how to merge an MR into multiple target branches at once:

• all merges should be pushed as one;
• each target branch may have a different commit within the MR which needs merged (occurs in the case where conflicts prevent a single commit from going into all branches).

Here's a diagram:

  <- I - I'
        /
<- N - M
      /
     B
    / \
<- R - R'

Where:

• I: the current HEAD of the target branch;
• R: the current HEAD of a backport branch;
• N: the topic for the target branch (if it were happening in isolation);
• B: the backported topic;
• M: a commit containing both the topic for the target branch and the backported topic in its history;
• I': the new HEAD into the target branch; and
• R': the new HEAD of the backport branch.

There may be multiple R branches for a given merge request.

Cc: @brad.king