Use *-base-rev platforms as base values when available in original results.

[gmierz]

This enables CompareView to use a "base-rev" build test when it's available in a push. See here for a sample try run of this platform: https://treeherder.mozilla.org/jobs?repo=try&revision=f7f43baf21021c4efc7fa34742da904faf9f03bb

You can test locally with a compare view link like this: /perfherder/compare?originalProject=try&newProject=try&newRevision=f7f43baf21021c4efc7fa34742da904faf9f03bb&framework=13&originalRevision=f7f43baf21021c4efc7fa34742da904faf9f03bb&page=1

[gmierz]

@beatrice-acasandrei I'm currently looking into making mach try perf run with a single push but an issue I hit is that it currently displays the two builds on different lines like so: https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=4711b3f52aee4f3246823c0b79f7912c3f939de6&newProject=try&newRevision=4711b3f52aee4f3246823c0b79f7912c3f939de6&framework=13&page=1

This patch is an attempt to resolve that issue. What do you think about this approach?