Bot support for github revisions

[vrigal]

Depends #3252

Analysis of a github PR requires the --github-repository=<path> to be set, linking to a local clone set up at the infra level.
I tested with TRY_TASK_ID=c9G2pWZiRm2bJsQ8ZRJNow and TRY_TASK_GROUP_ID=Gf_KQB-EQQSGNwIY5iV-gg.

[marco-c]

@vrigal looks like tests are failing

[La0]

I pushed a few patches to use the same naming convention for changesets & repos as on Phabricator.

This is now capable of loading issues found on a github PR:

export TRY_RUN_ID=0
export TRY_TASK_ID="IsqhgJyuQq2g2M6hmVoTag"
export TRY_TASK_GROUP_ID="TbyUh4mPQb-vlvzXOR9Vrw"
code-review-bot --conf=./conf-dev.yml --mercurial-repository=./cache/

But this lacks the actual git clone to build hashes and thus populate the backend with issues (only diff + revision is created now)

[La0]

Now the bot is able to clone base repo, set head repo as ref and checkout the precise revision.

Issues are finally published in the backend !