cmd/gitmirror: add 10 minute timeout to git fetch
cmd/gitmirror: add 10 minute timeout to git fetch

The Repo.fetch method tries to execute "git fetch" up to 3 times with
linear back-off. We don't expect the git fetch process to take more
than a few minutes unless something is very wrong. At that point,
it's better to abort and try again. So, add a 10 minute timeout.

This should help prevent fetch from being stuck and never retrying.

Fixes golang/go#35124

Change-Id: Ic06a552378f8bff1bea314288a3637548663a3ef
Reviewed-on: https://go-review.googlesource.com/c/build/+/203057
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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