The new Patcher class is responsible for taking a list of patches and

applying or reverting them, optionally committing them as well.
Consumers of Patcher are expected to provide the list of patches to

apply (each being a Patch object), the destination path (optionally),

any information known about the repository, and flags to initiate a

revert or squash of the patches.
If the patches are to be committed, they're then expected to call

prepare_for_commit(). Then, patch() can be called.
Patcher.patch() is a generator, taking care to apply each patch in

order and yielding a successful result for each or raising an exception.

Consumers can use the generator nature of this tool to help with

progress reporting.
By default, this interfaces with the local patch tool, supporting both

GNU Patch and Apple/BSD Patch. It takes care to examine output for both

tools and convert them into suitable responses. Subclasses can override

the methods to instead interface with SCM-specific patching tools, and

to even take care to apply or commit multiple patches in one go instead

of individually (which is needed for Mercurial).
Follow-up changes will begin integrating Patcher into the SCMs and

into rbt patch and rbt land.

Unit tests pass.