"Stage Hunk/Line For Commit" grayed out when having a custom diff

[wojnilowicz]

Steps to reproduce:

1. download and unpack partial-staging-and-custom-diff.zip
2. cd partial-staging-and-custom-diff
3. git gui
4. RMB on changed lines

Behavior:

"Stage Hunk/Line For Commit" menu entries are grayed out as in the image below

Expected behavior:

"Stage Hunk/Line For Commit" menu entries shouldn't be are grayed out.

Additional Info:

1. Version
   git-gui version 0.21.0.116.g0730a
   git version 2.43.0

   Tcl/Tk version 8.6.12
   Aspell 0.60.8, pl

2. The issue occurs when I have a ".gitattributes" file with following contents *.txt diff=myDiff and
   in the ".git/config" following contents

[diff "myDiff"]
	textconv=cat
	binary=false

3. git add -i respects binary=false set under [diff "myDiff"] and allows me to "Stage Hunk/Line For Commit" as in the image below
   
   It would not allow me to do so if binary=true. Meanwhile git gui doesn't allow me to do so in either case.