Add TakesFile to fish shell completion

The new `TakesFile` flag will be now consumed by the fish shell
completion generator.

Signed-off-by: Sascha Grunert <sgrunert@suse.com>

flag-gen/templates/cli_flags_generated.gotpl

@@ -60,6 +60,12 @@ func (f {{ $flag.Name }}Flag) GetValue() string {
 	{{ $flag.ValueString }}
 }
 
+// GetValue returns the flags value as string representation and an empty
+// string if the flag takes no value at all.
+func (f {{ $flag.Name }}Flag) GetTakesFile() bool {
+	{{ if eq $flag.TakesFile true }}return f.TakesFile{{ else }}return false{{ end }}
+}
+
 // {{ $flag.Name }} looks up the value of a local {{ $flag.Name }}Flag, returns
 // {{ $flag.ContextDefault }} if not found
 func (c *Context) {{ $flag.Name }}(name string) {{ if ne .ContextType "" }}{{ $flag.ContextType }}{{ else }}{{ $flag.Type }}{{- end }} {