Add exclude files support to the Perforce SCMClient.

The Perforce SCMClient now supports the -X flag for diff and post operations.
The -X parameter specifies a pattern to match against. To specify a pattern
against a depot, use `-X //depot/...` and to specify a relative (to the current
working directory) pattern, use `-X pattern`. A pattern that beings with a
single slash is interpreted as an absolute path.

Testing Done:
Generated diffs via `rbt diff -X` that excluded files. This was tested using
numbered changesets (both submitted and pending) and the default changeset
(i.e., open files)

Reviewed at https://reviews.reviewboard.org/r/6376/