Branch picker can get out of sync with selected branch

[BinaryMuse]

Description

The package can get into such a state that the branch picker and the active branch are not in sync.

Steps to Reproduce

1. Open a window with two project folders.
2. Open a file in the first project so that branch is shown.
3. Open a file in the second project so that the other repo's branch is shown.
4. Click the branch status bar item.

Expected behavior:

The drop-down matches the current branch.

Actual behavior:

The drop-down shows the branch from the last repo.

Reproduces how often:

100%

Versions

Atom 1.18.0-beta0