cmd/gerritbot: prevent duplicate posts, improve testability
cmd/gerritbot: prevent duplicate posts, improve testability

Gerritbot's duplicate suppression includes the full name of the
commenter, which means that if a commenter changes their name their
comments get reposted. This would be a mild irritation except that we
just changed gerritbot's name. Exclude the header from duplicate
detection.

To make it easier to test this change, add -single-pr, which ignores all
but one PR. Also do a TODO to move the config files to .config and cache
to .cache.

Fixes golang/go#41997.

Change-Id: I807efec79f423fe8fded5d08770919fde279b2e5
Reviewed-on: https://go-review.googlesource.com/c/build/+/371455
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build