Previously rbt patch would not work in a subdirectory or subdirectory

checkout. This was because the incorrect number of leading directories

were being stripped (due to our SVN diffs having absolute paths). We now

compute the correct number of leading directories to strip when applying

patches and they now apply correctly.
If a patch for SVN is generated in a different directory, files included

in the patch that are not under the current directory will not be

applied. If this occurs, the user will be informed.
This patch also fixes the issue of passing a value --px flag on the

commandline causing a formatting error.

Ran unit tests.
Manually tested the cases that gmyers outlined in

https://reviews.reviewboard.org/r/7095/. rbt patch works in all of

these cases.