cmdlib.sh: Use git timestamp for OSTree layers

That way the commit checksum itself is completely stable when
recomposing on the same git commit (modulo a dirty directory, but then
the checksum will change anyway).

Doesn't really matter for now (except for the good feelsies of always
getting the same commit hash between invocations), though it will if we
want to start recording more information about constituent layers in the
compose.

src/cmdlib.sh

@@ -239,6 +239,11 @@ runcompose() {
     local overridesdir=${workdir}/overrides
     local tmp_overridesdir=${TMPDIR}/override
     local override_manifest="${tmp_overridesdir}"/coreos-assembler-override-manifest.yaml
+
+    local git_timestamp="January 1 1970"
+    if [ -d "${configdir_gitrepo}/.git" ]; then
+        git_timestamp=$(git -C "${configdir_gitrepo}" show -s --format=%ci HEAD)
+    fi
     if [ -d "${overridesdir}" ] || [ -n "${ref_is_temp}" ] || [ -d "${configdir}/overlay" ]; then
         mkdir "${tmp_overridesdir}"
         cat > "${override_manifest}" <<EOF
@@ -256,7 +261,8 @@ EOF
     if [ -d "${configdir}/overlay" ]; then
         echo -n "Committing ${configdir}/overlay... "
         ostree commit --repo="${tmprepo}" --tree=dir="${configdir}/overlay" -b "${name}-overlay" \
-          --owner-uid 0 --owner-gid 0 --no-xattrs --no-bindings --parent=none
+          --owner-uid 0 --owner-gid 0 --no-xattrs --no-bindings --parent=none \
+          --timestamp "${git_timestamp}"
         cat >> "${override_manifest}" <<EOF
 ostree-layers:
   - ${name}-overlay
@@ -280,7 +286,8 @@ EOF
     if [ -d "${rootfs_overlay}" ]; then
         echo -n "Committing ${rootfs_overlay}... "
         ostree commit --repo="${tmprepo}" --tree=dir="${rootfs_overlay}" -b cosa-bin-overlay \
-          --owner-uid 0 --owner-gid 0 --no-xattrs --no-bindings --parent=none
+          --owner-uid 0 --owner-gid 0 --no-xattrs --no-bindings --parent=none \
+          --timestamp "${git_timestamp}"
           cat >> "${override_manifest}" << EOF
 ostree-override-layers:
   - cosa-bin-overlay