implement strip_outgoing() for Mercurial

[vladak]

When doing the changes for PR #4600, it occurred to me that a workaround would be to use the --strip-outgoing option of the opengrok-mirrror script however it turns out that mercurial.py does not implement strip_outgoing() function (unlike git.py).

[vladak]

For convenient modification, the strip Mercurial extension should be used; it needs to be explicitly enabled. This may involve creating a .hgrc configuration file to enable the extension.

[vladak]

advice from @vlmarek : hg --config extensions.mq= strip ...