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

mercurial/templatefilters.py

@@ -269,7 +269,7 b' def fill76(text):'
 def firstline(text):
     """Any text. Returns the first line of text."""
     try:
-        return text.splitlines(True)[0].rstrip(b'\r\n')
+        return text.splitlines()[0]
     except IndexError:
         return b''