Give better visual feedback when checking out the previous branch

[stefanhaller]

I find the command "Checkout previous branch" quite useful, and I use it a lot. However, I'm unhappy with the visual feedback, so this PR improves that a bit.

Previously, the feedback you got when pressing "-" was just a "Checking out..." status in the bottom line. This was both easy to miss if you are used to looking for an inline status in the branches panel, and it didn't provide information about which branch was being checked out, which can be annoying in very large repos where checking out takes a while, and you only see at the end if you are now on the right branch.

Improve this by trying to figure out which branch was the previously checked out one, and then checking it out normally so that you get an inline status next to it (as if you had pressed space on it). There are cases where this fails, e.g. when the previously checked out ref was a detached head, in which case we fall back to the previous behavior.

[stefanhaller]

Ping @kyu08 in case you want to have a look.

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for ba0e2d11	✅ 83.33%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (ba0e2d1)	Report Missing	Report Missing	Report Missing
Head commit (e585acc)	57756	50209	86.93%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4929)	18	15	83.33%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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Footnotes

1. Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct. ↩

[kyu08]

Thank you for letting me know!
I will test it later. 👍

[kyu08]

Thanks for implementing this!

I tried this with large repo and it looks clearer than as is. 👍

For the users who want to see what's changed visually, I put a screenshot.