File paths in diffs can come in any format. Windows paths, POSIX paths,

or UNC-like paths (or similar, such as for Perforce).
Review Bot has been assuming the file paths match the local environment,

which is generally safe, as POSIX is most common. However, it's an

assumption that could result in problems.
There are also problems with assuming the paths can be trusted. They may

be absolute, or may include a number of .. elements that could

result in issues modifying other files.
This change introduces some normalization and safeguards for working

with paths. A new get_path_platform() method inspects a path, trying

to determine if it's a Windows or POSIX path, and returns an enum value

representing it. Callers can then get the appropriate

os.path-equivalent module (ntpath or posixpath) from that value.
normalize_platform_path() makes use of this to convert a path to

something safe for use in patching files. It will split out any drive

letters or UNC hosts from a path, normalize any relative references

within the path, and then split/re-join it for the local system's

path separator.
It can optionally append this to a target directory as well, for

building an absolute path.
The resulting path is checked to ensure it's not referencing something

outside of the target directory. If it is, an exception is raised. This

mirrors similar behavior from GNU patch, which denies absolute or

relative references below the patch target directory.
This will be used for an upcoming replacement for the patching logic.

Unit tests pass.