cmdutil: create dirs for templated cat file output
Previously, the caller was responsible for creating the directory structure of
files written out using a path template. This is onerous, especially if the
exact filenames are not known upfront, but are being accessed via a matcher.
This patch changes things so that hg will attempt to create the appropriate
directories before writing the output file.
Differential Revision: https://phab.mercurial-scm.org/D1332