config: add new index.threads config setting
Add support for a new index.threads config setting which will be used to
control the threading code in do_read_index(). A value of 0 will tell the
index code to automatically determine the correct number of threads to use.
A value of 1 will make the code single threaded. A value greater than 1
will set the maximum number of threads to use.
For testing purposes, this setting can be overwritten by setting the
GIT_TEST_INDEX_THREADS=<n> environment variable to a value greater than 0.
Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>