The use-case for these font files is detailed in their 2013 changelog
entry:
> These "Unifont Sample" fonts contain combining circles, and four-digit
> hexadecimal glyphs for unassigned code points and Private Use Area glyphs.
> Because of the inclusion of combining cirlces, "Unifont Sample" font
> versions are only intended for illustrating individual glyphs, not for
> general-purpose writing.
What this means, in practice, is that since #550059 unifont *may* end-up
being picked rather than any other font that actually has a glyph,
instead of a replacement glyph.
And even if there was no proper glyph to use, a replacement glyph is
made on the fly anyway.
This regression can be checked with, for example, Font Awesome, on the
same config, Nixpkgs `b7c2ada94fe9..0e251e24a4f2`.
Before regression:
```
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-font-awesome-7.2.0/share/fonts/opentype/Font Awesome 7 Free-Solid-900.otf: Font Awesome 7 Free,Font Awesome 7 Free Solid:style=Solid,Regular
```
Current:
```
$ fc-list :charset=f244
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-font-awesome-7.2.0/share/fonts/opentype/Font Awesome 7 Free-Solid-900.otf: Font Awesome 7 Free,Font Awesome 7 Free Solid:style=Solid,Regular
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-unifont-17.0.05/share/fonts/opentype/unifont/unifont_sample.otf: Unifont Sample:style=Regular
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-unifont-17.0.05/share/fonts/X11/misc/unifont_sample.pcf.gz: Unifont Sample:style=Sans-Serif
```
After fix:
```
$ fc-list :charset=f244
/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-font-awesome-7.2.0/share/fonts/opentype/Font Awesome 7 Free-Solid-900.otf: Font Awesome 7 Free,Font Awesome 7 Free Solid:style=Solid,Regular
```
I do not have the time or energy to handle all of this. Let alone my
GitHub notifications. This PR is in good faith and does not refer to
anything community-related. I will keep on doing nixpkgs work but the
pile of shame (let's call it that) needs to be cleaned. Thanks to
@mweinelt which whom I had a discussion and who made me aware of this
issue.
Without the change the attempt to refetch patches fails with hash mismatch:
````
$ nix build --no-link -f. unity-test.patches --rebuild
error: hash mismatch in fixed-output derivation '/nix/store/h81wmb8js5h1g2is5bzisf6b9d36arxc-771.patch.drv':
specified: sha256-r8ldVb7WrzVwTC2CtGul9Jk4Rzt+6ejk+paYAfFlR5M=
got: sha256-viNwaqZ+hjIY4qnlLN55/TMzqmoNAgc1Eq5Yv17tr7c=
```
It happens because repository now adds one extra digit to abbreviated
hash:
```diff
diff -U0 before after
@@ -2 +2 @@
-index 6585129c..9489aef4 100644
+index 6585129c9..9489aef48 100644
```
Let's use `?full_index=1` to use full hash in diff output.
