vcs: sanitize diff context values (Issue #306)

- Updated Git repository implementation to ensure context falls within
0 to 2**31-1 range (inclusive) when fetching a diff.
- Added tests for Git repositories for checking passed-in negative and
overflowing contexts (for the --unified option).
- Updated Mercurial repository implementation to ensure context is not
negative when fetching a diff.
- Added tests for Mercurial repositories for checking passed-in
negative context (for the --unified option).