over-eager population of git branch menu when RStudio is focused

[kevinushey]

System details

RStudio Edition : Desktop
RStudio Version : v1.2.907
OS Version      : macOS 10.13.6
R Version       : 3.5.1

Steps to reproduce the problem

Open a project containing a large number of Git branches (e.g. RStudio itself), and ensure the Git pane is open. Then, try unfocusing and refocusing the IDE window.

Describe the problem in detail

On each focus, RStudio re-populates the Git Branch menu, and this operation is expensive when many branches exist.

Describe the behavior you expected

The menu should be populated on-demand, when clicking on the branch menu.

Notes

Note that in this case, we were stuck waiting for ~3s while the menu was populated.

[jmcphers]

Performance regression (keeping in 1.2)