This is part of #3956 and is for viewing changes as compared to any branch (ex: master).

Changes:

• create a feature flag (ideally we don't change anything in the HistorySideBar component)
• copy change (history -> compare, changes -> commit [may not be reflected in mocks, talk to @niik])
• branch selecting input with new icon in input
• list of branches on branch selecting input click (if it's empty) which can be "x"ed out
• clicking on a branch shows commits ahead of the target branch
• filtering

User Flow to check functionality:

• click on compare tab and see the commits you made on your branch + all commits from where you branched off of (existing behavior)
• click into the "compare a branch..." input and see a list of branches just like you would for switching branches if you clicked on the "current branch" menu item
• click on "master" and "master" becomes the text in the input and you see all the commits you have ahead of "master"
• press "x" in the branch input and see list of branches again
• select another branch, ex: "new-branch" and "new-branch" becomes the text in the input and you see all the commits you have ahead of "new-branch"
• press "x" in the branch input and see list of branches again
• start typing the name of an existent branch in "compare a branch..." input
• see all branches filtered with same behavior as we have for the branches container
• select a new target branch, ex: "filtered-branch" and "filtered-branch" becomes the text in the input and you see all the commits you have ahead of "filtered-branch"
• press "x" in the branch input and see list of branches again
• see the compare tab return to default state

Other pieces:

• ahead/behind counts for each branch is in [RBB] Ahead/Behind Branch Commit Counts #3968
• toggle for behind/ahead or any behind info in [RBB] Ahead/Behind Toggle #3997

Notes:
@donokuda said this may change soon

Before:

(branch list in app for reference)

After:
(default view)

(select branch options)

(selecting branch)

(filtering branch)