Add support for -X to git SCMClient

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.

Testing Done:
- Unit tests pass.
- Generated diffs with excluded files via `rbt diff` with git, git-svn,
  and git-p4.

Reviewed at https://reviews.reviewboard.org/r/6339/