cli: fmt -check should return early on diff

The `nomad fmt -check` command incorrectly writes to file because we didn't
return before writing the file on a diff. Fix this bug and update the command
internals to differentiate between the write-to-file and write-to-stdout code
paths, which are activated by different combinations of options and flags.

The docstring for the `-list` and `-write` flags is also unclear and can be
easily misread to be the opposite of the actual behavior. Clarify this and fix
up the docs to match.

This changeset also refactors the tests quite a bit so as to make the test
outputs clear when something is incorrect.

.changelog/16174.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+cli: Fixed a bug where `nomad fmt -check` would overwrite the file being checked
+```

command/fmt.go

@@ -37,7 +37,8 @@ type FormatCommand struct {
 	check        bool
 	checkSuccess bool
 	recursive    bool
-	write        bool
+	writeFile    bool
+	writeStdout  bool
 	paths        []string
 
 	stdin io.Reader
@@ -47,27 +48,31 @@ func (*FormatCommand) Help() string {
 	helpText := `
 Usage: nomad fmt [flags] paths ...
 
-	Formats Nomad agent configuration and job file to a canonical format.
-	If a path is a directory, it will recursively format all files
-	with .nomad and .hcl extensions in the directory.
-	
-	If you provide a single dash (-) as argument, fmt will read from standard
-	input (STDIN) and output the processed output to standard output (STDOUT).
+  Formats Nomad agent configuration and job file to a canonical format.
+  If a path is a directory, it will recursively format all files
+  with .nomad and .hcl extensions in the directory.
+
+  If you provide a single dash (-) as argument, fmt will read from standard
+  input (STDIN) and output the processed output to standard output (STDOUT).
 
 Format Options:
 
-	-list=false
-		Don't list the files, which contain formatting inconsistencies.
+  -check
+	Check if the files are valid HCL files. If not, exit status of the
+	command will be 1 and the incorrect files will not be formatted. This
+    flag overrides any -write flag value.
 
-	-check
-		Check if the files are valid HCL files. If not, exit status of the command
-		will be 1 and the incorrect files will not be formatted.
+  -list
+	List the files which contain formatting inconsistencies. Defaults
+	to -list=true.
 
-	-write=false
-		Don't overwrite the input files.
+  -recursive
+	Process files in subdirectories. By default only the given (or current)
+	directory is processed.
 
-	-recursive
-		Process also files in subdirectories. By default only the given (or current) directory is processed.
+  -write
+	Overwrite the input files. Defaults to -write=true. Ignored if the input
+    comes from STDIN.
 `
 
 	return strings.TrimSpace(helpText)
@@ -100,7 +105,7 @@ func (f *FormatCommand) Run(args []string) int {
 	flags := f.Meta.FlagSet(f.Name(), FlagSetClient)
 	flags.Usage = func() { f.Ui.Output(f.Help()) }
 	flags.BoolVar(&f.check, "check", false, "")
-	flags.BoolVar(&f.write, "write", true, "")
+	flags.BoolVar(&f.writeFile, "write", true, "")
 	flags.BoolVar(&f.list, "list", true, "")
 	flags.BoolVar(&f.recursive, "recursive", false, "")
 
@@ -121,11 +126,18 @@ func (f *FormatCommand) Run(args []string) int {
 	if len(flags.Args()) == 0 {
 		f.paths = []string{"."}
 	} else if flags.Args()[0] == stdinArg {
-		f.write = false
+		f.writeStdout = true
+		f.writeFile = false
 		f.list = false
 	} else {
 		f.paths = flags.Args()
 	}
+	if f.check {
+		f.writeFile = false
+	}
+	if !f.list && !f.writeFile {
+		f.writeStdout = true
+	}
 
 	f.fmt()
 
@@ -251,19 +263,20 @@ func (f *FormatCommand) processFile(path string, r io.Reader) {
 			f.Ui.Output(path)
 		}
 
-		if f.write {
+		if f.check {
+			f.checkSuccess = false
+		}
+
+		if f.writeFile {
 			if err := os.WriteFile(path, out, 0644); err != nil {
 				f.appendError(fmt.Errorf("Failed to write file %s: %w", path, err))
 				return
 			}
 		}
 
-		if f.check {
-			f.checkSuccess = false
-		}
 	}
 
-	if !f.list && !f.write {
+	if f.writeStdout {
 		f.Ui.Output(string(out))
 	}
 }