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".

mercurial/util.py

@@ -2308,9 +2308,9 b' def removeauth(u):'
     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