bug#73142: Erroring libsixel package's bash completions for img2sixel

[elaexuotee]

It appears that share/bash-compltion/bash_completion performs an
`unset -f have' at the end of the script. This means that only the scripts
under /etc/bash_completion.d/ have access to `have', but said directory doesn't
even exist in our case.

That said, `have' is deprecated in favor of `_have' in 2.11, anyway. I have let
upstream libsixel know, but we could work around the issue with a
single-character patch:

diff --git a/converters/shell-completion/bash/img2sixel 
b/converters/shell-completion/bash/img2sixel
index 028651b..d96a326 100644
--- a/converters/shell-completion/bash/img2sixel
+++ b/converters/shell-completion/bash/img2sixel
@@ -1,6 +1,6 @@
 # bash completion for img2sixel

-have img2sixel &&
+_have img2sixel &&
 _img2sixel()
 {
     local cur prev