x/build/watcher: crashing after finding commit in two branches

[bradfitz]

Yo, @adg, the watcher is crash-looping with:

2016/07/20 13:52:30 Watcher exiting after failure: found commit we already knew about: 1d2ca9e30c22bc9e8cd0b21dff58367443696c91("doc/go1.7.html: start sentence on a new line"); first seen on dev.garbage, now on master

/cc @quentinmit

[gopherbot]

CL https://golang.org/cl/25086 mentions this issue.