This patch adds a `capture_log` flag that will allow a user to capture the logs produced when running the `push_to_try` methods. Currently, we use `subprocess.check_call` which causes hg to hang when we attempt to redirect and gather stdout. Using `subprocess.run` is better as it lets us capture the logs, but the log output is very slow and can easily lead people to believe that hg is hanging when it's not. This results in corrupted repos. Using Popen, the logs are output slower than `check_call`, but faster than `run` so you know something is happening. The speed at which these logs are printed is also why I have this log capturing behind a flag.
This functionality will be used in the `./mach try perf` selector to capture the child revisions produced for the try task config file changes and allow us to produce a PerfCompare link to provide the user.
Differential Revision: https://phabricator.services.mozilla.com/D155980
e27dd67d9d