util: checknlink should remove file it creates if an exception occurs

There's no reason to leave the file behind.

mercurial/util.py

@@ -1341,6 +1341,10 b' def checknlink(testfile):'
     try:
         posixfile(f1, 'w').close()
     except IOError:
+        try:
+            os.unlink(f1)
+        except OSError:
+            pass
         return False
 
     f2 = testfile + ".hgtmp2"