Whitelist "python_bin_reldir.txt".

the revert mechanism for the Python bundle patch relies on the resitual
"python_bin_reldir.txt" file existing. Apparently "bot_update" purges
this file as part of its depot_tools checkout, breaking the revert
mechanism. Whitelist it so that this stops happening.

TBR=tandrii@chromium.org
BUG=chromium:741650
TEST=None

Change-Id: Id6b7c966b6fa3f5084be5e1906b3e10d38ab8b6c
Reviewed-on: https://chromium-review.googlesource.com/568530
Reviewed-by: Daniel Jacques <dnj@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>

.gitignore

@@ -33,6 +33,7 @@
 
 # Ignore locations where third-party tools are placed during bootstrapping.
 /python*_bin
+/python_bin_reldir.txt
 /git_bin
 /git-*_bin
 /svn_bin