build: Fix git submodule fetching with builddir != srcdir

We were running the git submodule update in builddir, instead of srcdir,
which might have lead to submodules not updating, and submodule tags
being reverted to older versions in some contributions.

https://bugzilla.gnome.org/show_bug.cgi?id=773085

autogen.sh

@@ -11,6 +11,8 @@ test -z "$srcdir" && srcdir=.
     exit 1
 }
 
+pushd $srcdir
+
 # Fetch submodules if needed
 if test ! -f src/gvc/Makefile.am || test ! -f data/theme/gnome-shell-sass/COPYING;
 then
@@ -19,6 +21,8 @@ then
 fi
 git submodule update
 
+popd
+
 which gnome-autogen.sh || {
     echo "You need to install gnome-common from GNOME Git (or from"
     echo "your OS vendor's package manager)."