util: rename argument of isatty()

util: rename argument of isatty()

In general, "fd" is a file descriptor, but isatty() expects a file object.
We should call it "fp" or "fh".

--- a/mercurial/util.py	Sat Nov 21 16:21:52 2015 +0900
+++ b/mercurial/util.py	Sun Dec 13 18:48:35 2015 +0900
@@ -2308,9 +2308,9 @@
     u.user = u.passwd = None
     return str(u)
 
-def isatty(fd):
+def isatty(fp):
     try:
-        return fd.isatty()
+        return fp.isatty()
     except AttributeError:
         return False