Tidy up Rx in PullRequestStatusBarManager

[jcansdale]

• Use CombineLatest to combine the latest repository and PR session
• Use Throttle(TimeSpan.FromSeconds(1)) to get rid of jitter when repo / branch change
• Use Forget(log) to log exception (rather than a try catch on async method)

[codecov]

Codecov Report

Merging #1999 into master will decrease coverage by 1.43%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1999      +/-   ##
==========================================
- Coverage   41.69%   40.25%   -1.44%     
==========================================
  Files         373      405      +32     
  Lines       16129    17344    +1215     
  Branches     2212     2391     +179     
==========================================
+ Hits         6725     6982     +257     
- Misses       8893     9834     +941     
- Partials      511      528      +17

Impacted Files	Coverage Δ
...ineReviews/Services/PullRequestStatusBarManager.cs	0% <0%> (ø)	⬆️
...nlineReviews/Services/PullRequestSessionManager.cs	82.23% <0%> (-1.02%)	⬇️
...c/GitHub.InlineReviews/Tags/InlineCommentTagger.cs	79.64% <0%> (-0.89%)	⬇️
...nlineReviews/Services/PullRequestSessionService.cs	16.7% <0%> (-0.41%)	⬇️
...rc/GitHub.VisualStudio/Commands/OpenLinkCommand.cs	0% <0%> (ø)
...Hub.VisualStudio/Commands/ShowGitHubPaneCommand.cs	0% <0%> (ø)
...o/Commands/GoToSolutionOrPullRequestFileCommand.cs	0% <0%> (ø)
src/GitHub.VisualStudio/Services/UsageTracker.cs	92.53% <0%> (ø)
.../GitHub.VisualStudio/Services/ShowDialogService.cs	0% <0%> (ø)
...Hub.VisualStudio/Commands/ShowMessageBoxCommand.cs	0% <0%> (ø)
... and 26 more