Currently, perf-tar*-src-pkg only uses 'git archive', but it is better
to make it work without relying on git.

The file, HEAD, which saves the commit hash, will be included in the
tarball only when the source tree is managed by git. The git tree is
more precisely checked; it has been copied from scripts/setlocalversion.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>