gvfs: ensure that the version is based on a GVFS tag

Since we really want to be based on a `.vfs.*` tag, let's make sure that
there was a new-enough one, i.e. one that agrees with the first three
version numbers of the recorded default version.

This prevents e.g. v2.22.0.vfs.0.<some-huge-number>.<commit> from being
used when the current release train was not yet tagged.

It is important to get the first three numbers of the version right
because e.g. Scalar makes decisions depending on those (such as assuming
that the `git maintenance` built-in is not available, even though it
actually _is_ available).

Signed-off-by: Johannes Schindelin <johasc@microsoft.com>

GIT-VERSION-GEN

@@ -15,10 +15,15 @@ if test -f version
 then
 	VN=$(cat version) || VN="$DEF_VER"
 elif { test -d "${GIT_DIR:-.git}" || test -f .git; } &&
-	VN=$(git describe --match "v[0-9]*" HEAD 2>/dev/null) &&
+	VN=$(git describe --match "v[0-9]*vfs*" HEAD 2>/dev/null) &&
 	case "$VN" in
 	*$LF*) (exit 1) ;;
 	v[0-9]*)
+		if test "${VN%%.vfs.*}" != "${DEF_VER%%.vfs.*}"
+		then
+			echo "Found version $VN, which is not based on $DEF_VER" >&2
+			exit 1
+		fi
 		git update-index -q --refresh
 		test -z "$(git diff-index --name-only HEAD --)" ||
 		VN="$VN-dirty" ;;