largefiles: simplify lfutil.writehash

This was unnecessarily verbose: there is no need to unlink the file
when we open it for write anyway, and there is no need to check if the
file exists after we created it.

hgext/largefiles/lfutil.py

@@ -393,17 +393,8 b' def readhash(filename):'
 
 def writehash(hash, filename, executable):
     util.makedirs(os.path.dirname(filename))
-    if os.path.exists(filename):
-        os.unlink(filename)
-    wfile = open(filename, 'wb')
-
-    try:
-        wfile.write(hash)
-        wfile.write('\n')
-    finally:
-        wfile.close()
-    if os.path.exists(filename):
-        os.chmod(filename, getmode(executable))
+    util.writefile(filename, hash + '\n')
+    os.chmod(filename, getmode(executable))
 
 def getexecutable(filename):
     mode = os.stat(filename).st_mode