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

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.