Makefile: merge main-version and git-version earlier

Makefile: merge main-version and git-version earlier

With GNU Make 4.3 on both ArchLinux, and VoidLinux,
GITVERION is always empty because of bad substitution.
Change '\#' to simply '#' can fix it,
but we don't need that complation.

We can merge DRACUT_MAIN_VERSION and GITVERSION into DRACUT_FULL_VERSION.
Because, GITVERSION will be attached back to DRACUT_MAIN_VERSION in all
situation.

While we're at it, detect if we're in git worktree by:
limiting GIT_CEILING_DIRECTORIES to parent directory of
dracut's top level directory; instead of checking for .git directory,
in order to support git-worktree, in such case, .git will be a file, see
gitrepository-layout(5)