cmdutil: extract function checking if pattern should be taken as stdin/out

This will be used in commands.cat().

mercurial/cmdutil.py

@@ -610,6 +610,10 b' def makefilename(repo, pat, node, desc=N'
         raise error.Abort(_("invalid format spec '%%%s' in output filename") %
                          inst.args[0])
 
+def isstdiofilename(pat):
+    """True if the given pat looks like a filename denoting stdin/stdout"""
+    return not pat or pat == '-'
+
 class _unclosablefile(object):
     def __init__(self, fp):
         self._fp = fp
@@ -635,7 +639,7 b' def makefileobj(repo, pat, node=None, de'
 
     writable = mode not in ('r', 'rb')
 
-    if not pat or pat == '-':
+    if isstdiofilename(pat):
         if writable:
             fp = repo.ui.fout
         else: