t6421: fix test to work when repo dir contains d0
The `grep` statement in this test looks for `d0.*<string>`, attempting to filter to only show lines that had tabular output where the 2nd column had `d0` and the final column had a substring of [`git -c `]`fetch.negotiationAlgorithm`. These lines also have `child_start` in the 4th column, but this isn't part of the condition. A subsequent line will have `d1` in the 2nd column, `start` in the 4th column, and `/path/to/git/git -c fetch.negotiationAlgorihm` in the final column. If `/path/to/git/git` contains the substring `d0`, then this line is included by `grep` as well as the desired line, leading to an effective doubling of the number of lines, and test failures. Tighten the grep expression to require `d0` to be surrounded by spaces, and to have the `child_start` label. Signed-off-by: Kyle Lippincott <spectral@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>