fake_repos: Log stderr when Git fails to start

Bug: 962263
Change-Id: I71824bf30ff6d735d07e70e8b1cd054530d8985b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1607844
Auto-Submit: Raul Tambre <raul@tambre.ee>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Raul Tambre <raul@tambre.ee>

testing_support/fake_repos.py

@@ -108,7 +108,9 @@ def wait_for_port_to_bind(host, port, process):
     sock.close()
   # The process failed to bind. Kill it and dump its ouput.
   process.kill()
-  logging.error('%s' % process.communicate()[0])
+  stdout, stderr = process.communicate()
+  logging.debug('%s' % stdout)
+  logging.error('%s' % stderr)
   assert False, '%d is still not bound' % port