feat(LogSearch): Allow aborting search

[StemCll]

This Pull Request fixes/closes https://github.com/extrawurst/gitui/issues/1860 Tested on Linux github repo searching anything - it's big enough to make filtering take time.

It changes the following:

• Adds cancellation_flag to AsyncCommitFilterJob
• cancels LogSearch when pressed escape

I followed the checklist:

• [ ] I added unittests
• [x] I ran make check without errors
• [x] I tested the overall application
• [ ] I added an appropriate item to the changelog