make_file: always return a fresh file handle that can be closed

Currently, cmdutil.make_file() will return a freshly made file handle,
except when given a pattern of '-'. If callers would want to close the
handle, they would have to make sure that it's neither sys.stdin or
sys.stdout. Instead, returning a duplicate of either of the two
ensures that make_file() lives up to its name and creates a new
file handle regardless of the input.

mercurial/cmdutil.py

@@ -233,7 +233,8 b' def make_file(repo, pat, node=None,'
     writable = 'w' in mode or 'a' in mode
 
     if not pat or pat == '-':
-        return writable and sys.stdout or sys.stdin
+        fp = writable and sys.stdout or sys.stdin
+        return os.fdopen(os.dup(fp.fileno()), mode)
     if hasattr(pat, 'write') and writable:
         return pat
     if hasattr(pat, 'read') and 'r' in mode:

tests/test-hook.t

@@ -73,8 +73,8 b' test generic hooks'
   [1]
   $ hg cat b
   pre-cat hook: HG_ARGS=cat b HG_OPTS={'rev': '', 'decode': None, 'exclude': [], 'output': '', 'include': []} HG_PATS=['b'] 
+  b
   post-cat hook: HG_ARGS=cat b HG_OPTS={'rev': '', 'decode': None, 'exclude': [], 'output': '', 'include': []} HG_PATS=['b'] HG_RESULT=0 
-  b
 
   $ cd ../b
   $ hg pull ../a