This mode is designed for repositories with very large indexes,
and aims at reducing the time it takes to repeatedly write these
indexes.
In this mode, the index is split into two files, $GIT_DIR/index
and $GIT_DIR/sharedindex.<SHA-1>. Changes are accumulated in
$GIT_DIR/index, the split index, while the shared index file
contains all index entries and stays unchanged.
All changes in the split index are pushed back to the shared
index file when the number of entries in the split index reaches
a level specified by the splitIndex.maxPercentChange config
variable (see git-config(1)).
Each time a new shared index file is created, the old shared
index files are deleted if their modification time is older than
what is specified by the splitIndex.sharedIndexExpire config
variable (see git-config(1)).
To avoid deleting a shared index file that is still used, its
modification time is updated to the current time every time a new
split index based on the shared index file is either created or
read from.
