After 2f6b50d by @jnavila, asciidoctor builds don't work:

$ git checkout 2f6b50d146016205047834165a81810d7849efce
$ asciidoctor progit.asc
asciidoctor: WARNING: book/contributors.asc: line 8: include file not found: ~/progit2/book/contributors.txt

Prior to 2f6b50d, they do:

$ git checkout 2f6b50d146016205047834165a81810d7849efce^
$ asciidoctor progit.asc
$

This rendered incorrect the instructions that were in README.asc at the time of that commit (and that remain in README.asc today):

Use the following commands:

$ asciidoctor progit.asc

Either the instructions in README.asc should be corrected, or else the build process using asciidoctor should be fixed. I would prefer the latter, but either would be preferable to the current situation. Thanks :)