Git (and the git-svn and git-p4 backends) now support the -X/--exclude

commandline option and the EXCLUDE_PATTERNS .reviewboardrc option.
Add unit tests for excluding files from Git diffs.
Rename exclude_files/EXCLUDE_FILES to exclude_patterns/EXCLUDE_PATTERNS

to reflect the actual behaviour of -X.

Unit tests pass.
Generated diffs with excluded files via rbt diff with git, git-svn,

and git-p4.