Adjust tests so that they won't scream when the default initial
branch name is changed to 'main'.
* js/default-branch-name-part-4-minus-1:
t1400: prepare for `main` being default branch name
tests: prepare aligned mentions of the default branch name
t9902: prepare a test for the upcoming default branch name
t3200: prepare for `main` being shorter than `master`
t5703: adjust a test case for the upcoming default branch name
t6200: adjust suppression pattern to also match "main"
tests: start moving to a different default main branch name
t9801: use `--` in preparation for default branch rename
fmt-merge-msg: also suppress "into main" by default