cmd/gitmirror: add tests and update documentation

Also add closing HTML tags to the /debug/watcher page.

Change-Id: I487874802a2a79ae442ca996f00ef2f04ef6dc16
Reviewed-on: https://go-review.googlesource.com/37221
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed