RBTools is pretty heavily dependent on a source checkout for most

operations. For some tools, this is a problem, as they have very little

need for the checkout (at least in certain modes) but not having a

checkout means overriding a lot of logic. rbt patch --print, for

instance, tries to work without a source tree, but the recent DVCS work

broke this due to how fragile and unclear the workarounds were.
This change adds support in initialize_scm_tool() and

scan_usable_client() for specifying that a working checkout is

optional. These will still attempt to find a repository checkout, but

will return None if one can't be found, rather than erroring out.
rbt patch --print has been fixed up to use this, in order to restore

the ability to work outside a source tree.

Tested that rbt patch --print was usable outside a source tree,

but that a source tree was still required and checked when not

using --print.
Unit tests pass.