cmd/cue: make cue fmt --check print files immediately

This makes `cue fmt --check` immediately print the path of each
badly formatted file to stdout when they're discovered, instead
of displaying all files at the end of the run.

This makes the command feel slightly more responsive and quicker,
as the command "hangs" for less time.

Signed-off-by: Noam Dolovich <noam.tzvi.dolovich@gmail.com>
Change-Id: Ifb0e707b5f57e15bf44269d6de367dcce397a0e5
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193776
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/fmt.go

@@ -62,8 +62,10 @@ func newFmtCmd(c *Command) *cobra.Command {
 			cfg.Format = opts
 			cfg.Force = true
 
+			var foundBadlyFormatted bool
 			check := flagCheck.Bool(cmd)
-			var badlyFormattedFiles []string
+			cwd, _ := os.Getwd()
+			stdout := cmd.OutOrStdout()
 
 			for _, inst := range builds {
 				if inst.Err != nil {
@@ -127,25 +129,22 @@ func newFmtCmd(c *Command) *cobra.Command {
 					}
 
 					if check && !bytes.Equal(formatted.Bytes(), original) {
-						badlyFormattedFiles = append(badlyFormattedFiles, file.Filename)
+						foundBadlyFormatted = true
+
+						if file.Filename != "-" {
+							f := file.Filename
+							var path string
+							path, err = filepath.Rel(cwd, f)
+							if err != nil {
+								path = f
+							}
+							fmt.Fprintln(stdout, path)
+						}
 					}
 				}
 			}
 
-			if check && len(badlyFormattedFiles) > 0 {
-				cwd, _ := os.Getwd()
-				stdout := cmd.OutOrStdout()
-				for _, f := range badlyFormattedFiles {
-					if f == "-" {
-						continue
-					}
-
-					relPath, err := filepath.Rel(cwd, f)
-					if err != nil {
-						relPath = f
-					}
-					fmt.Fprintln(stdout, relPath)
-				}
+			if check && foundBadlyFormatted {
 				return ErrPrintedError
 			}