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

mercurial/cmdutil.py

@@ -3029,6 +3029,11 b' def cat(ui, repo, ctx, matcher, basefm, '
         if fntemplate:
             filename = makefilename(repo, fntemplate, ctx.node(),
                                     pathname=os.path.join(prefix, path))
+            # attempt to create the directory if it does not already exist
+            try:
+                os.makedirs(os.path.dirname(filename))
+            except OSError:
+                pass
         with formatter.maybereopen(basefm, filename, opts) as fm:
             data = ctx[path].data()
             if opts.get('decode'):

tests/test-cat.t

@@ -126,7 +126,6 b' Test behavior of output when directory s'
   $ echo a > foo/a
   $ hg add foo/a
   $ hg commit -qm "add foo/a"
-  $ mkdir output
   $ hg cat --output "output/%p" foo/a
-  abort: No such file or directory: output/foo/a
-  [255]
+  $ cat output/foo/a
+  a