genindex.sh: take optional project name to increase speed
If an optional project name is given, then only the entry
for that project name will be updated.
Running git config unnecessarily thousands of times just to
add or remove one entry makes project creation and deletion
unnecessarily slow. Instead we can use join to extract the
previously computed values for all the entries we're not
updating to save time.
It's not so much the actual time it takes to retrieve the
config entry as it is the disk waiting to access files that
may not be in the cache and also the churn of creating all
those git processes.