cmd: fix completion of remotes

The previous behavior of the remotes completion was that only
alphanumeric characters were allowed in a remote name. This limitation
has been lifted somewhat by #2985, which also allowed an underscore.

With the new implementation introduced in this commit, the completion of
the remote name has been simplified: If there is no colon (":") in the
current word, then complete remote name. Otherwise, complete the path
inside the specified remote. This allows correct completion of all
remote names that are allowed by the config (including - and _).
Actually it matches much more than that, even remote names that are not
allowed by the config, but in such a case there already would be a wrong
identifier in the configuration file.

With this simpler string comparison, we can get rid of the regular
expression, which makes the completion multiple times faster. For a
sample benchmark, try the following:

     # Old way
     $ time bash -c 'for _ in {1..1000000}; do
         [[ remote:path =~ ^[[:alnum:]]*$ ]]; done'

     real    0m15,637s
     user    0m15,613s
     sys     0m0,024s

     # New way
     $ time bash -c 'for _ in {1..1000000}; do
         [[ remote:path != *:* ]]; done'

     real    0m1,324s
     user    0m1,304s
     sys     0m0,020s

cmd/help.go

@@ -45,7 +45,7 @@ __rclone_custom_func() {
         else
             __rclone_init_completion -n : || return
         fi
-        if [[ $cur =~ ^[[:alnum:]_]*$ ]]; then
+        if [[ $cur != *:* ]]; then
             local remote
             while IFS= read -r remote; do
                 [[ $remote != $cur* ]] || COMPREPLY+=("$remote")
@@ -54,7 +54,7 @@ __rclone_custom_func() {
                 local paths=("$cur"*)
                 [[ ! -f ${paths[0]} ]] || COMPREPLY+=("${paths[@]}")
             fi
-        elif [[ $cur =~ ^[[:alnum:]_]+: ]]; then
+        else
             local path=${cur#*:}
             if [[ $path == */* ]]; then
                 local prefix=${path%/*}