Currently, running git cl presubmit fails without python 2 installed.
This CL replaces instances of 'python' in fetch_test.py with the generic
sys.executable and fixes a small error testRebaseUpdate due to an extra
period.

Bug: 1289839
Change-Id: Ib5802545af70fe0886448397fa76058bd2036527
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3407864Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>