x/build: remove redundant [...] on build.golang.org for branches

[randall77]

When you view, for example, the dev.ssa branch on build.golang.org, all commits* are prefixed with [dev.ssa]. Together with the fact that there's not a lot of space for the commit title, you end up with the build dashboard looking like:

[dev.ssa] cmd/compile: ...
[dev.ssa] cmd/compile: ...
[dev.ssa] cmd/compile: ...
[dev.ssa] cmd/compile: ...
[dev.ssa] cmd/compile: ...

which is pretty uninformative. It's similarly redundant for all the other branches. Could we elide the [branch] prefix? It's useful for email but for the dashboard we've already filtered on this field.

*except merges from master

@bradfitz @adg

[rsc]

Maybe we should also make the commit message column a little wider too.