Make vcs-tag do something useful for non-developer mode as well

When building packages of arbitrary commits of systemd-stable,
distributors might want to include a git sha of the exact commit
they're on. Let's extend vcs-tag a little to make this possible.

If we're on a commit matching a tag, don't generate a git sha at all.
If we're not on a commit matching a tag, generate a vcs tag as usually.
However, if we're not in developer mode, don't append a '^' if the tree
is dirty to accomodate package builds applying various patches to the
tree which shouldn't be considered as "dirty" edits.

tools/vcs-tag.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+set -e
+
+MODE="$1"
+
+if ! [[ -d .git ]] || git describe --tags --exact-match &>/dev/null; then
+    exit 0
+fi
+
+if [[ "$MODE" == "developer" ]]; then
+    DIRTY="--dirty=^"
+else
+    DIRTY=""
+fi
+
+echo "-g$(git describe --abbrev=7 --match="" --always $DIRTY)"