cat: test output path behvaior when target path does not exist

In the next patch, we will change the behavior of the cat command to create
output file parent directories before attempting to write the output file.
This patch documents the current unfortunate behavior.

Differential Revision: https://phab.mercurial-scm.org/D1331

tests/test-cat.t

@@ -119,3 +119,14 b' Environment variable visibility can be e'
   $ PATTERN='t4' hg log -r '.' -T "{envvars % '{key} -> {value}\n'}" \
   >                 --config "experimental.exportableenviron=PATTERN"
   PATTERN -> t4
+
+Test behavior of output when directory structure does not already exist
+
+  $ mkdir foo
+  $ 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]