cat: record the current behavior of wildcard matches in subrepos

Mercurial subrepos support any match patterns.

mercurial/help/subrepos.txt

@@ -90,7 +90,7 b' Interaction with Mercurial Commands'
 :archive: archive does not recurse in subrepositories unless
     -S/--subrepos is specified.
 
-:cat: cat currently only handles exact file matches in subrepos.
+:cat: Git subrepositories only support exact file matches.
     Subversion subrepositories are currently ignored.
 
 :commit: commit creates a consistent snapshot of the state of the

tests/test-subrepo.t

@@ -1071,6 +1071,18 b' Prepare a repo with subrepo'
     "path": "sub/repo/foo"
    }
   ]
+
+ non-exact match:
+
+  $ hg cat -T '{path}\n' 'glob:**'
+  .hgsub
+  .hgsubstate
+  sub/repo/foo (glob)
+  $ hg cat -T '{path}\n' 're:^sub'
+  sub/repo/foo (glob)
+
+ missing subrepos in working directory:
+
   $ mkdir -p tmp/sub/repo
   $ hg cat -r 0 --output tmp/%p_p sub/repo/foo
   $ cat tmp/sub/repo/foo_p