I've verified and guarantee, that provided cyrillic_input_stream file contains common `git fast-export` UTF-8 encoded stream from valid git repo, containing exactly one patch named 'Южноэфиопский грач увёл мышь за хобот на съезд ящериц' with exactly one file with cyrillic filename 'Панграмма.txt' with exactly one line of text 'Широкая электрификация южных губерний даст мощный толчок подъёму сельского хозяйства' and is valid for recreating exactly the same git repo by piping it on `git fast-import`, which I twice tested manually (on 2020-06-29, when creating the patch, and now).
I'm also forgot to mention, that issue only exists for `darcs convert import`, while `darcs convert export` was and is totally OK both with common Darcs 2.14.2 and custom repo with patch from Ganesh.
I was explaining him all the situation at #darcs on IRC on 2020-06-09 , so he made this patch especially for me :) for testing.
Other people on #darcs are also very nice, friendly and helpful, and even guided me with building Darcs from source! :)
Sad, but even this didn't help, so he adviced me to open an issue and provide a test case for it, which I wasn't able to do until now (was ill).
Do you think it would help for me to provide similar input stream sample from valid fresh Darcs repo, created with the same UTF-8 cyrillic patch/file name/contents?