Add input flag to skip bisection

[ebyerly]

Blocked pending https://github.com/envoyproxy/nighthawk/pull/1472/changes

Core change adds skip_bisection flag to only check if we can add the latest Envoy commit and end.

QoL tweaks:

• Commit message includes Envoy commit date and a link to see the relevant Envoy commit.
• move git add . operation to end of Envoy commit integration - if the process makes it to that step, those changes are good and can be stored.
• remove extracting the Github username - Git UI already prints an equivalent message to stdout indicating the user can follow a link to create a pull request.
• remove separate tracking of latest_envoy_commit and replace with a check that the latest commit in the range is equal to the Nighthawk current Envoy commit and HEAD.
• remove separate tracking of latest_passing_commit_index and incrementally update best_envoy_commit and first_non_trivial_commit