repo-settings: create feature.manyFiles setting

repo-settings: create feature.manyFiles setting

The feature.manyFiles setting is suitable for repos with many
files in the working directory. By setting index.version=4 and
core.untrackedCache=true, commands such as 'git status' should
improve.

While adding this setting, modify the index version precedence
tests to check how this setting overrides the default for
index.version is unset.

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