tests: slightly modify a linkrev test to prepare for expanding it The test case checks that the copy tracing code doesn't get confused by linkrevs when walking a file's ancestors. This patch chnages the test slightly so a second commit is grafted, thus producing a second "bad" linkrev. I'll use this in the next patch to demonstrate a bug. Differential Revision: https://phab.mercurial-scm.org/D6321