Make taskmaster consumption of failed tasks more FIFO-ey

Summary:
Ref T1049. See discussion in D7745. We have some specific interest in this for D7745, but generally we want to consume tasks with expired leases in roughly FIFO order, just like we consume new tasks in roughly FIFO order. Currently, when we select an expired task we order them by `id`, but this is the original insert order, not lease expiration order. Instead, order by `leaseExpires`.

This query is actually much better than the old one was, since the WHERE part is `leaseExpries < VALUE`.

Test Plan: Ran `EXPLAIN` on the query. Ran a taskmaster in debug mode and saw it lease new and expired tasks successfully.

Reviewers: hach-que, btrahan

Reviewed By: hach-que

CC: aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7746
Fix the exception when watching the first commit of a mercurial repo

Summary: Most checks were actually in place, but `ExecFuture` throws a `CommandException` which wasn't taken into account.

Test Plan: look at the first command and no longer saw an exception. Also, other commits worked as well.

Reviewers: richardvanvelzen

Reviewed By: richardvanvelzen

CC: krisbuist, Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7730
Raise a better error for malformed `git ls-tree`

Summary: Ref T4159. See T4159 for discussion.

Test Plan: Faked the error and generated a reasonable error message.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4159

Differential Revision: https://secure.phabricator.com/D7744
Show a queue utilization statistic in the Daemon console

Summary:
This came up recently in a discussion with @lifeihuang, and then tangentally with @hach-que. Make it easier for users to get a sense of whether they might need to add more daemons. Although we've improved the transparency of daemons, it's not easy for non-experts to determine at a glance how close to overflowing the queue is.

This number is approximate, but should be good enough for determining if your queue is more like 25% or 95% full.

If this goes over, say, 80%, it's probably a good idea to think about adding a couple of daemons. If it's under that, you should generally be fine.

Test Plan: {F88331}

Reviewers: btrahan, hach-que, lifeihuang

Reviewed By: btrahan

CC: hach-que, lifeihuang, aran, chad

Differential Revision: https://secure.phabricator.com/D7747
plug octocat icon to landing button

Test Plan: look at shiny button

Reviewers: epriestley, chad, #blessed_reviewers

Reviewed By: chad

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7749
Implement "Wait for Previous Builds" build step

Summary: This adds a build step which will block a build from continuing if there are previous builds of the build plan still running.

Test Plan: Configured a build plan with a wait of 60 seconds and a "wait for previous builds", then started a build.  While that was still building, reconfigured the plan to have a wait time of 3 seconds, started it, and saw it move into the "Waiting" status.  When the 60 second build finished, both builds passed.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D7745