templates: make `firstline` filter not keep '\v', '\f' and similar In b288b4bb8448 (hide some functions behind lambdas, so demandload is useful., 2006-02-28), `x.splitlines(1)[0]` was replaced by `x.splitlines(1)[0].rstrip('\r\n')`, i.e. stripping trailing '\r' and '\n'. Combined with the "truthy" `1` passed to `splitlines()` to get it to keep line endings, that results in e.g. trailing '\v' (Line Tabulation) and '\f' (Form Feed) being preserved. I can't see why one would want that, and I doubt that was the intention; I suspect the author just didn't think to instead remove the `1` argument. Perhaps they thought the 1 being passed there - added by themselves in a7e416bf3c1d (improve templating., 2006-02-27) - was to limit the number of splits to 1 (i.e. thinking about it as `maxsplit=1` rather than `keepends=1`). Differential Revision: https://phab.mercurial-scm.org/D12403