Use proper workdir and srcdir.

- Use this.srcdir for 'copy' method's source checkout.
- Keep track of the workdir, don't assume its called 'build'
same fix for git. use "old_workdir" as in svn.py
Fixed CVS, Git source step "copy" method tests.
Release notes: Git and CVS silently changed workdir
Merge branch 'source-method-copy-srcdir' of git://github.com/jpommerening/buildbot