Exclude patterns are now interpreted as relative to the current

working directory.
Add rbtools.utils.diffs:normalize_patterns which normalizes patterns,

which are interpreted as relative to the current working directory,

into patterns matching only absolute paths.
Simplify logic of rbtools.utils.diffs:filter_diff to use

filename_match_any_patterns instead of looping through patterns.
The rbtools.utils.diffs functions filename_match_any_patterns,

filter_diff, and remove_filenames_matching_patterns now take an

extra parameter: base_dir. This argument indicates the directory

that the files are located in. If this argument is not provided, it is

interpreted as the current working directory.
Add GitClient._get_root_directory which finds the absolute path of

the git repository.
All SCMClients that support file exclusion normalize their exclude

patterns. This is not done outside of the respective diff methods

because Perforce exclude patterns require special treatment for depot

file patterns.
The SVN SCMClient no longer prepends / to patterns as it no longer

matches the / in the Index: line as the file path as they are now

interpreted as relative file paths.
Update documentation for rbt diff and rbt post to indicate that

-X specifies relative exclude patterns. Also add a note that Perforce

can specify depot-specific exclude patterns.

Unit tests pass.
Ran rbt diff -X in SVN, CVS, Git, and Bazaar repositories with

relative paths. Files were excluded as expected.