For some reason, after 10 failed tries yesterday, just now it worked (without making the changes in the PR cited above). The initial download part of the process took about 30 seconds.
FYI, my localhost server (XAMPP) is running cURL 7.45.0 with OpenSSL 1.0.2n.
I have a suspicion that my ISP might have been having problems yesterday and the connection to Github might just have been lost part way through each time.
One curious thing: on my dashboard page, the ‘At a Glance’ widget shows “ClassicPress 1.0.0+migration.20190305 running RCG3 theme.” I would have expected it to just say “1.0.0”.
EDIT: after using the “If you need to re-install version 1.0.0, you can do so here” option on the updates page, the at a glance widget changed to “ClassicPress 1.0.0 running RCG3 theme.”