plugins/percol: use _command_exists

Addresses #1632

And use `_log_warning`.

Alsö, code style cleanup: quote things, handle unbound parameters, &c.

Alsö alsö, short-circuit if not installed or inadequate _Bash_ version.

plugins/available/percol.plugin.bash

@@ -1,3 +1,4 @@
+# shellcheck shell=bash
 cite about-plugin
 about-plugin 'Search&Select history with percol'
 
@@ -12,24 +13,26 @@ about-plugin 'Search&Select history with percol'
 # Usage
 ## C-r to search&select from history
 
-_replace_by_history() {
-    if command -v tac>/dev/null; then
+_command_exists percol || return
+
+if [[ ${BASH_VERSINFO[0]} -lt 4 ]]; then
+   _log_warning "You have to upgrade Bash to Bash v4.x to use the 'percol' plugin."
+   _log_warning "Your current Bash version is $BASH_VERSION."
+   return
+else
+    bind -x '"\C-r": _replace_by_history'
+fi
+
+function _replace_by_history() {
+	local HISTTIMEFORMAT= # Ensure we can parse history properly
+    if _command_exists tac
+	then
         alias _tac=tac
     else
         alias _tac="tail -r"
     fi
-    local l=$(HISTTIMEFORMAT= history | _tac | sed -e 's/^\ *[0-9]*\ *//' | percol --query "$READLINE_LINE")
-    READLINE_LINE="$l"
+	#TODO: "${histlines[@]/*( )+([[:digit:]])*( )/}"
+    local l="$(history | _tac | sed -e 's/^\ *[0-9]*\ *//' | percol --query "${READLINE_LINE:-}")"
+    READLINE_LINE="${l}"
     READLINE_POINT=${#l}
 }
-
-
-if command -v percol>/dev/null; then
-    current_version=${BASH_VERSION%%[^0-9]*}
-    if [ $current_version -lt 4 ]; then
-       echo -e "\033[91mWarning: You have to upgrade Bash to Bash v4.x to use the 'percol' plugin.\033[m"
-       echo -e "\033[91m         Your current Bash version is $BASH_VERSION.\033[m"
-    else
-        bind -x '"\C-r": _replace_by_history'
-    fi
-fi