repl: tests show difference (in color where available)

compiler/tests/repl/.gitignore

@@ -1 +1,3 @@
 run
+*.repl.result.txt
+*.repl.expected.txt

compiler/tests/repl/runner/runner.v

@@ -25,6 +25,20 @@ pub fn full_path_to_v() string {
 	return vexec
 }
 
+fn find_working_diff_command() ?string {
+	for diffcmd in ['colordiff', 'diff', 'colordiff.exe', 'diff.exe'] {
+		p := os.exec('$diffcmd --version') or { continue }
+		if p.exit_code == 0 { return diffcmd }
+	}
+	return error('no working diff command found')
+}
+
+fn diff_files( file_result, file_expected string ) string {
+	diffcmd := find_working_diff_command() or { return err }
+	diff := os.exec('$diffcmd   --minimal  --text   --unified=2  $file_result  $file_expected') or { return 'found diff command "$diffcmd" does not work' }
+	return diff.output
+}
+
 pub fn run_repl_file(wd string, vexec string, file string) ?string {
 	fcontent := os.read_file(file) or {	return error('Could not read file $file') }
 	content := fcontent.replace('\r', '')		
@@ -43,11 +57,18 @@ pub fn run_repl_file(wd string, vexec string, file string) ?string {
 	result := r.output.replace('\r','').replace('>>> ', '').replace('>>>', '').replace('... ', '').all_after('Use Ctrl-C or `exit` to exit\n').replace(wd, '' )
 
 	if result != output {
+		file_result   := '${file}.result.txt'
+		file_expected := '${file}.expected.txt'
+		os.write_file( file_result, result )
+		os.write_file( file_expected, output )
+		diff := diff_files( file_result, file_expected )
 		return error('Difference found in REPL file: $file
 ====> Got      :
 |$result|
 ====> Expected :
 |$output|
+====> Diff     :
+$diff
 		')
 	} else {
 		return 'Repl file $file is OK'

examples/.gitignore

@@ -8,3 +8,4 @@
 /nbody
 /spectral
 /database/mysql
+/hello_v_js