diff: list only changed, added and modified files

Use --diff-filter=ACM option to `git diff` command in order to only list
Added (A), Changed (C) and Modified (M) files.

There is no reason to lint deleted files and it was causing some
problemas as reported on https://github.com/grvcoelho/lint-diff/issues/3

Author: grvcoelho

src/lint-diff.js

@@ -30,7 +30,7 @@ const linter = new CLIEngine()
 const formatter = linter.getFormatter()
 
 const getChangedFiles = pipeP(
-  commitRange => exec('git', ['diff', commitRange, '--name-only']),
+  commitRange => exec('git', ['diff', commitRange, '--name-only', '--diff-filter=ACM']),
   prop('stdout'),
   split('\n'),
   filter(endsWith('.js')),