Make mysql completions work with log-in (#7397)

Complete databases using credentials specified on the commandline.

share/functions/__fish_complete_mysql.fish

@@ -0,0 +1,25 @@
+function __fish_mysql_query -a query
+    argparse -i 'u/user=' 'P/port=' 'h/host=' 'p/password=?' 'S/socket=' -- (commandline -po)
+    set -l mysql_cmd mysql
+    for flag in u P h S
+        if set -q _flag_$flag
+            set -l flagvar _flag_$flag
+            set -a mysql_cmd -$flag $$flagvar
+        end
+    end
+    if [ -n "$_flag_p" ]
+        set -a mysql_cmd -p$_flag_p
+    end
+    echo $query | $mysql_cmd 2>/dev/null
+end
+
+function __fish_complete_mysql_databases
+    __fish_mysql_query 'show databases'
+end
+
+function __fish_complete_mysql
+    set -l command $argv[1]
+
+    complete -c $command -s D -l database -x -d 'The database to use' -a '(__fish_complete_mysql_databases)'
+    complete -c $command -a '(__fish_complete_mysql_databases)'
+end