config: close file even if we fail to read it

If we get an exception from cfg.read(), we would not close the file
before this patch. This patch uses a context manager to make sure we
close it.

Differential Revision: https://phab.mercurial-scm.org/D7626

mercurial/ui.py

@@ -434,16 +434,16 b' class ui(object):'
                 return
             raise
 
+        with fp:
         cfg = config.config()
         trusted = sections or trust or self._trusted(fp, filename)
 
         try:
             cfg.read(filename, fp, sections=sections, remap=remap)
-            fp.close()
         except error.ParseError as inst:
             if trusted:
                 raise
-            self.warn(_(b"ignored: %s\n") % stringutil.forcebytestr(inst))
+                self.warn(_(b'ignored: %s\n') % stringutil.forcebytestr(inst))
 
         if self.plain():
             for k in (