Make autogen.sh set the default format.subjectPrefix

CONTRIBUTING requests that people do this, but it's a lot easier if we
just set it up by default for them.

v2: Update CONTRIBUTING about this happening (suggested by Petri).
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Petri Latvala <petri.latvala@intel.com> (v1)

CONTRIBUTING

@@ -12,11 +12,11 @@ A short list of contribution guidelines:
 
   Please use --subject-prefix="PATCH i-g-t" so that i-g-t patches are easily
   identified in the massive amount mails on intel-gfx. To ensure this is always
-  done just run
+  done, autogen.sh will run:
 
     git config format.subjectprefix "PATCH i-g-t"
 
-  from within your i-g-t git checkout.
+  on its first invocation.
 
 - intel-gpu-tools is MIT lincensed and we require contributions to follow the
   developer's certificate of origin: http://developercertificate.org/

autogen.sh

@@ -16,6 +16,9 @@ fi
 autoreconf -v --install || exit 1
 cd $ORIGDIR || exit $?
 
+git config --local --get format.subjectPrefix >/dev/null 2>&1 ||
+    git config --local format.subjectPrefix "PATCH i-g-t"
+
 if test -z "$NOCONFIGURE"; then
         $srcdir/configure "$@"
 fi