xmllint fixer must read from stdin - not buffer filename (#4884)

The current xmllint fixer reads and formats the file that a buffer is
associated with from disk instead of accepting input from stdin. This
has the side effect that if the filename is changed in the buffer, but
not saved yet, the fixer discards all the pending changes and replaces
the buffer contents with the formatted text from the file contents on
disk.

test/test-files/xml/dummy.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0"?>
+<xml>
+  <test id="12"/>
+</xml>