[#6821] Handle errors in threaded git LCDs without indefinitely blocking request

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6821] Refactored common last commit logic up and removed merge commit skipping

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6821] Remove any possibility of indefinite blocking and make test more clear

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6821] Remove old last_commit_ids method and fix handling of merge commits in git

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6821] Remove LCD dependency on DiffInfoDoc

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
[#6821] Fixed failing tests due to last commit refactor

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>