On Unix all files are opened in binary mode. On Windows linefeeds are translated to CRLF corrupting files that are meant to be binary. This is the reason of HgGit failure - it uses Dulwich library that creates corrupted Git index file, because on Windows it is opened without os.O_BINARY flag. Developers do not know anything about it, because they develop on Unix. This could be prevented if we supply a note about how to preserve crossplatform behavior.
https://bugs.edge.launchpad.net/dulwich/+bug/557585

I've attached a patch that adds required note. Please revise it. It should clearly apply over your changes in r80069. I reworded description towards Python users without C background, but I am still worried that they need a POSIX background to understand umask part.