filemerge: rename _formatconflictmarker() since it formats a label `_formatconflictmarker()`'s name made me think that it would create something like `<<<<<<<` and maybe some more stuff after it, but it's actually just the label that comes after on the same line. So let's rename it to `_formatlabel()`. That's better aligned with the variables the result is assigned it. Differential Revision: https://phab.mercurial-scm.org/D11801