add test-git-ignore.py, to port the 'clean' buildbot test to git

add .gitignore to match .darcs-boringfile, mostly

misc/build_helpers/test-git-ignore.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python
+
+import sys
+from subprocess import Popen, PIPE
+
+cmd = ["git", "status", "--porcelain"]
+p = Popen(cmd, stdout=PIPE)
+output = p.communicate()[0]
+print output
+if output == "":
+    sys.exit(0)
+sys.exit(1)
+
+