www_coderepo: use OnDestroy to render summary view

www_coderepo: use OnDestroy to render summary view

This lets us get rid of a /bin/sh process and allows us us to
rely on Qspawn to parallelize git commands.

Special treatment of the OnDestroy object is necessary to keep
its scope limited for MockHTTP.  Neither the generic `plackup'
HTTP server and nor our -httpd/-netd needed this scope
limitation.  As a result, summary() is now called inside an
anonymous sub to keep the memory overhead of the anonymous sub
itself as small as possible.  Avoiding anonymous subs entirely
would be preferable for memory savings, but it's necessary for
PSGI.