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).