travis: run tests with GIT_TEST_SPLIT_INDEX

travis: run tests with GIT_TEST_SPLIT_INDEX

Split index mode only has a few dedicated tests, but as the index is
involved in nearly every git operation, this doesn't quite cover all the
ways repositories with split index can break.  To use split index mode
throughout the test suite a GIT_TEST_SPLIT_INDEX environment variable
can be set, which makes git split the index at random and thus
excercises the functionality much more thoroughly.

As this is not turned on by default, it is not executed nearly as often
as the test suite is run, so occationally breakages slip through.  Try
to counteract that by running the test suite with GIT_TEST_SPLIT_INDEX
mode turned on on travis.

To avoid using too many cycles on travis only run split index mode in
the linux-gcc target only.  The Linux build was chosen over the Mac OS
builds because it tends to be much faster to complete.

The linux gcc build was chosen over the linux clang build because the
linux clang build is the fastest build, so it can serve as an early
indicator if something is broken and we want to avoid spending the extra
cycles of running the test suite twice for that.

Helped-by: Lars Schneider <larsxschneider@gmail.com>
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>