scalar: use index.skipHash=true for performance

scalar: use index.skipHash=true for performance

The index.skipHash config option has been set to 'false' by Scalar since
4933152cbb (scalar: enable path-walk during push via config, 2025-05-16)
but that commit message is trying to communicate the exact opposite:
that the 'true' value is what we want instead. This means that we've
been disabling this performance benefit for Scalar repos
unintentionally.

Fix this issue before we add justification for the config options set in
this list.

Oddly, enabling index.skipHash causes a test issue during 'test_commit'
in one of the Scalar tests when GIT_TEST_SPLIT_INDEX is enabled (as
caught by the linux-test-vars build). I'm fixing the test by disabling
the environment variable, but the issue should be resolved in a series
focused on the split index.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>