t6423: fix missed staging of file in testcases 12i,12j,12k

t6423: fix missed staging of file in testcases 12i,12j,12k

Commit 806f83287f8d (t6423: test directory renames causing
rename-to-self, 2021-06-30) introduced testcase 12i-12k but omitted
staging one of the files and copy-pasted that mistake to the other
tests.  This means the merge runs with an unstaged change, even though
that isn't related to what is being tested and makes the test look more
complicated than it is.

The cover letter for the series associated with the above commit (see
Message-ID: pull.1039.git.git.1624727121.gitgitgadget@gmail.com) noted
that these testcases triggered two bugs in merge-recursive but only one
in merge-ort; in merge-recursive these testcases also triggered a
silent deletion of the file in question when it shouldn't be deleted.
What I didn't realize at the time was that the deletion bug in merge-ort
was merely being sidestepped by the "relevant renames" optimization but
can actually be triggered.  A subsequent commit will deal with that
additional bug, but it was complicated by the mistaken forgotten
staging, so this commit first fixes that issue.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>