add flag to avoid version reset

[rogpeppe]

When we've previously checked out a version of a dependency in a gohack module dir and we want to keep that version instead of switching to the version specified in the go.mod file, it would be nice to have a flag to do that instead of running gohack and then switching back.

[corona10]

How about creating a new command?

$> gohack commit or gohack commit --all
$> gohack commit <specific_package>

[rogpeppe]

What would you expect that commit command to do?