tools: support `v fmt -w -backup file.v`

cmd/tools/vfmt.v

@@ -26,6 +26,7 @@ struct FormatOptions {
 	is_noerror bool
 	is_verify  bool // exit(1) if the file is not vfmt'ed
 	is_worker  bool // true *only* in the worker processes. NB: workers can crash.
+	is_backup  bool // make a `file.v.bak` copy *before* overwriting a `file.v` in place with `-w`
 }
 
 const (
@@ -53,6 +54,7 @@ fn main() {
 		is_debug: '-debug' in args
 		is_noerror: '-noerror' in args
 		is_verify: '-verify' in args
+		is_backup: '-backup' in args
 	}
 	if term_colors {
 		os.setenv('VCOLORS', 'always', true)
@@ -249,6 +251,10 @@ fn (foptions &FormatOptions) post_process_file(file string, formatted_file_path
 	}
 	if foptions.is_w {
 		if is_formatted_different {
+			if foptions.is_backup {
+				file_bak := '${file}.bak'
+				os.cp(file, file_bak) or {}
+			}
 			os.mv_by_cp(formatted_file_path, file) or { panic(err) }
 			eprintln('Reformatted file: $file')
 		} else {

cmd/v/help/fmt.txt

@@ -18,6 +18,9 @@ Options:
   -l      List files whose formatting differs from vfmt.
 
   -w      Write result to (source) file(s) instead of to stdout.
+  
+  -backup In combination with `-w`, copy the original `file.v` to a `file.v.bak` backup,
+          before overwriting the original source file.
 
   -debug  Print the kinds of encountered AST statements/expressions on stderr.