Run try with a specific parent commit

[Kobzol]

I often want to test multiple related changes in PRs that can affect performance and compare the performance effect of these changes. The usual workflow is like this:

1. Commit A
2. Run try build + perf run. The try build will use parent commit P1.
3. Commit B
4. Run try build + perf run. The try build will use parent commit P2.

The problem with this approach is that between the commits A and B are built using try, the parent commit from master might have changed already and therefore P1 != P2. Master changes multiple times per day, so it can happen even if you schedule the second try build right after the previous one has finished. This makes it impossible to precisely compare the performance of A and B.

It would be nice to have the option to say something like @bors try parent=<commit sha> so that when I run the second try build, I can manually specify parent P1 to make sure that I am comparing against the same baseline. Or even something like @bors try parent=last to make the bot use the latest parent commit, so that I don't have to look it up.

I checked the code briefly and I think that it could be done by optionally replacing this with a commit sha specified in the try command (?).