Update the bash completion script (#5301)

Some functions used by the bash completion script have become
deprecated. This was causing the test suite to break on faster-moving
distributions (e.g. Arch Linux and Gentoo). The issue is only noticeable
because the tests for the completion function explicitly disallow
compatibility implementations from being loaded. Rather than allowing
compatibility implementations, I've added some basic logic to the
completion script so that older and newer versions of `bash-completion`
can be supported simultaneously.

beets/ui/completion_base.sh

@@ -31,7 +31,7 @@
 # plugins dynamically
 #
 # Currently, only Bash 3.2 and newer is supported and the
-# `bash-completion` package is required.
+# `bash-completion` package (v2.8 or newer) is required.
 #
 # TODO
 # ----
@@ -46,7 +46,30 @@
 # * Support long options with `=`, e.g. `--config=file`. Debian's bash
 #   completion package can handle this.
 #
+# Note that 'bash-completion' v2.8 is a part of Debian 10, which is part of
+# LTS until 2024-06-30.  After this date, the minimum version requirement can
+# be changed, and newer features can be used unconditionally.  See PR#5301.
+#
 
+if [[ ${BASH_COMPLETION_VERSINFO[0]} -ne 2 \
+   || ${BASH_COMPLETION_VERSINFO[1]} -lt 8 ]]; then
+  echo "Incompatible version of 'bash-completion'!"
+  return 1
+fi
+
+# The later code relies on 'bash-completion' version 2.12, but older versions
+# are still supported.  Here, we provide implementations of the newer functions
+# in terms of older ones, if 'bash-completion' is too old to have them.
+
+if [[ ${BASH_COMPLETION_VERSINFO[1]} -lt 12 ]]; then
+  _comp_get_words() {
+    _get_comp_words_by_ref "$@"
+  }
+
+  _comp_compgen_filedir() {
+    _filedir "$@"
+  }
+fi
 
 # Determines the beets subcommand and dispatches the completion
 # accordingly.
@@ -54,7 +77,7 @@ _beet_dispatch() {
   local cur prev cmd=
 
   COMPREPLY=()
-  _get_comp_words_by_ref -n : cur prev
+  _comp_get_words -n : cur prev
 
   # Look for the beets subcommand
   local arg
@@ -99,7 +122,7 @@ _beet_complete() {
     completions="${flags___common} ${opts} ${flags}"
     COMPREPLY+=( $(compgen -W "$completions"  -- $cur) )
   else
-    _filedir
+    _comp_compgen_filedir
   fi
 }
 
@@ -114,12 +137,12 @@ _beet_complete_global() {
       ;;
     -l|--library|-c|--config)
       # Filename completion
-      _filedir
+      _comp_compgen_filedir
       return
       ;;
     -d|--directory)
       # Directory completion
-      _filedir -d
+      _comp_compgen_filedir -d
       return
       ;;
   esac