test-lib: allow setting the index format version

test-lib: allow setting the index format version

Allow adding a TEST_GIT_INDEX_VERSION variable to config.mak to set the
index version with which the test suite should be run.

If it isn't set, the default version given in the source code is
used (currently version 3).

To avoid breakages with index versions other than [23], also set the
index version under which t2104 is run to 3.  This test only tests
functionality specific to version 2 and 3 of the index file and would
fail if the test suite is run with any other version.

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>