Code Quality: Improved performance of Git operations

[hishitetsu]

Changes

• SemaphoreSlim is used instead of ThreadWithMessageQueue
• Only GetRepositoryHead limits the number of concurrent operations. Limiting others may cause problems such as delay in displaying the item list.

Resolved / Related Issues

• Were these changes approved in an issue or discussion with the project maintainers? In order to prevent extra work, feature requests and changes to the codebase must be approved before the pull request will be reviewed. This prevents extra work for the contributors and maintainers.
  Closes #issue...

Validation
How did you test these changes?

• Did you build the app and test your changes?
• Did you check for accessibility? You can use Accessibility Insights for this.
• Did you remove any strings from the en-us resource file?
  • Did you search the solution to see if the string is still being used?
• Did you implement any design changes to an existing feature?
  • Was this change approved?
• Are there any other steps that were used to validate these changes?
  1. Navigate to a Git repository from outside the repository.
  2. Navigate immediately to a subfolder of the repository.
  3. The delay in displaying items, which sometimes occurred especially in environments with weak networks, is now fixed.

[yaira2]

LGTM