Fix 'update_pnacl_tool_revisions.py: support git instead of svn'
Fix 'update_pnacl_tool_revisions.py: support git instead of svn'

My previous CL:
  https://codereview.chromium.org/826863007/

Had a bug when trying to figure out if the user had provided the hash or not. Fix it.

R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/864113003

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client