Make sure rustbot only builds the most recent revision of a branch

[catamorphism]

I noticed just now that the linux2 bot was building a version of the "incoming" branch that had 4 more revisions that were newer than it. For branches like incoming that don't get pushed to with -f, the bot should pick the newest one first (or only build the newest one).

This might require a way to configure rustbot to know which branches may have had force pushes (and obviously that depends on people with commit access to not do a mistaken force-push, too...)

[eholk]

I wonder if Rustbot could just always fast forward (but not merge) before rebuilding. Maybe that could automatically deal with the difference between -f branches and otherwise.

[graydon]

This should be filed on the rustbot repo. You have access to it, yes?

(I generally concur, but it's a question of doing a branch-by-branch flag; some branches should build everything pushed, some should only build the most-recent)

[graydon]

Oh. I guess we have A-rustbot here. Nevermind.

[brson]

rustbot dead