This is part of #3956 and provides the option to bring changes from a target branch into your branch using a merge button

User Flow to check functionality:

• click on compare tab, click into the "compare a branch..." input, and click on "master" and see:
  • Behind (x) in the toggle selected with the commits that are missing from your branch that exist in the target branch
  • a merge button saying Merge into my-branch-name
  • supporting text This will merge x commits from (target branch name)
• click on Ahead (y) in the toggle where y is the # of commits your branch is ahead of "master" and see:
  • no merge button
• click "x" in the branch input, select another target branch, and see:
  • the option to merge a different target branch's commits into your branch in the default behind view only
• click back to Behind (x) and click on the merge button and see:
  • commits disappear from Behind area and toggle becomes Behind (0)
  • Ahead is not affected
• click "x" in the branch input and focus out of the input and see:
  • the recently merged commits included in the history now

Changes:

• merge button with text Merge into (my-branch-name)
• supporting text below merge button with a count matching x in the Behind (x) toggle
• clicked Merge into (my-branch-name) merges those changes into your branch
• disabled merge button when there is nothing to merge (ex: after completing a merge)
• reverting on conflict + error message

Other pieces:

• toggling is in [RBB] Ahead/Behind Toggle #3997
• predicting merge conflicts will happen in a later issue

Before (shows "ahead" by default):

After: