A regression in git-blame prints an incorrect error message which causes
this test case to fail. Alter the test to only check the start of the
string, until the bug is fixed upstream.

Bug: 737688
Change-Id: I4045cb8792d8abe984215c7198e213b23e9f6f5d
Reviewed-on: https://chromium-review.googlesource.com/567778
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>