autogen: Set a default subject prefix for patches

Per discussion at XDC2015, we want this so we can easily distinguish
which module a patch is for. There's no way to set this in the
server-side config, so setting a default at autogen time is about the
best we can do.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit a722d617a0)

autogen.sh

@@ -9,6 +9,9 @@ cd "$srcdir"
 autoreconf --force -v --install || exit 1
 cd "$ORIGDIR" || exit $?
 
+git config --local --get format.subjectPrefix ||
+    git config --local format.subjectPrefix "PATCH xserver"
+
 if test -z "$NOCONFIGURE"; then
     exec "$srcdir"/configure "$@"
 fi