Respect a GIT_INDEX_VERSION environment variable, when a new index is
initialized. Setting the environment variable will not cause existing
index files to be converted to another format, but will only affect
newly written index files. This can be used to initialize repositories
with index-v4.
Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>