dist: archive HEAD instead of HEAD^{tree}

Update `git archive` tree-ish argument from `HEAD^{tree}` to `HEAD`. By
using a commit (rather than tree) reference, the commit hash will be stored
as an extended pax header, extractable git `git get-tar-commit-id`.

The intended use-case for this change is building `git` from the output of
`make dist` - in combination with the ability to specify a fallback
`GIT_BUILT_FROM_COMMIT`, a user can extract the commit ID used to build the
archive and set it as `GIT_BUILT_FROM_COMMIT`. The result is fully-populated
information for the commit hash in `git version --build-options`.

Signed-off-by: Victoria Dye <vdye@github.com>

Author: vdye

Diff for: Makefile

@@ -3707,7 +3707,7 @@ dist: git-archive$(X) configure
 	@$(MAKE) -C git-gui TARDIR=../.dist-tmp-dir/git-gui dist-version
 	./git-archive --format=tar \
 		$(GIT_ARCHIVE_EXTRA_FILES) \
-		--prefix=$(GIT_TARNAME)/ HEAD^{tree} > $(GIT_TARNAME).tar
+		--prefix=$(GIT_TARNAME)/ HEAD > $(GIT_TARNAME).tar
 	@$(RM) -r .dist-tmp-dir
 	gzip -f -9 $(GIT_TARNAME).tar