Documentation/Makefile: fix interrupted builds of user-manual.xml

Documentation/Makefile: fix interrupted builds of user-manual.xml

Unlike gcc, asciidoc does not atomically write its output file or
delete it when interrupted.  If it is interrupted in the middle of
writing an XML file, the result will be truncated input for xsltproc.

	XSLTPROC user-manual.html
	user-manual.xml:998: parser error : Premature end of data in t

Take care of this case by writing to a temporary and renaming it when
finished.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>