Add intermediate build products to .gitignore

Temporaries such as configure.ac+ and Documentation/*.xml+
sometimes remain after an interrupted build.  Tell git not to
track them.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

.gitignore

@@ -168,6 +168,7 @@ git.spec
 *.exe
 *.[aos]
 *.py[co]
+*+
 config.mak
 autom4te.cache
 config.cache