fix Bash completion on macOS (#2074)

* fix Bash completion on macOS

The Homebrew-provided bash-completion is version 1.x,
which doesn't provide _init_completion. We add a standard
shim instead.

* Add entry to CHANGELOG

* Update CHANGELOG.md

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>

* review: avoid calling _split_longopt repeatedly

Co-authored-by: Ville Skyttä <ville.skytta@iki.fi>

CHANGELOG.md

@@ -7,6 +7,8 @@
 
 ## Bugfixes
 
+- Fix bash completion on bash 3.x and bash-completion 1.x. See #2066 (@joshpencheon)
+
 ## Other
 
 ## Syntaxes

assets/completions/bat.bash.in

@@ -2,9 +2,22 @@
 
 # Requires https://github.com/scop/bash-completion
 
+# Macs have bash3 for which the bash-completion package doesn't include
+# _init_completion. This is a minimal version of that function.
+__bat_init_completion()
+{
+	COMPREPLY=()
+	_get_comp_words_by_ref "$@" cur prev words cword
+}
+
 _bat() {
-	local cur prev words cword split
-	_init_completion -s || return 0
+	local cur prev words cword split=false
+	if declare -F _init_completion >/dev/null 2>&1; then
+		_init_completion -s || return 0
+	else
+		__bat_init_completion -n "=" || return 0
+		_split_longopt && split=true
+	fi
 
 	if [[ ${words[1]-} == cache ]]; then
 		case $prev in