Merge branch 'tg/index-v4-format'

* tg/index-v4-format:
read-cache: add index.version config variable
test-lib: allow setting the index format version
introduce GIT_INDEX_VERSION environment variable

diff --cc Documentation/config.txt
Simple merge

diff --cc Makefile
Simple merge

diff --cc read-cache.c
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge