format: Don't wrap space-prefixed diag details

Diagnostic detail lines sometimes contain lines which include commands
suggested for the user to execute. By convention, these start with
leading whitespace to indicate that they are not prose.

This commit changes the diagnostic formatter to wrap each line of the
detail separately, and skips word wrapping for lines prefixed with
space. This prevents ugly and confusing wrapping of long command lines.

command/format/diagnostic.go

@@ -175,11 +175,17 @@ func Diagnostic(diag tfdiags.Diagnostic, sources map[string][]byte, color *color
 	}
 
 	if desc.Detail != "" {
-		detail := desc.Detail
 		if width != 0 {
-			detail = wordwrap.WrapString(detail, uint(width))
+			lines := strings.Split(desc.Detail, "\n")
+			for _, line := range lines {
+				if !strings.HasPrefix(line, " ") {
+					line = wordwrap.WrapString(line, uint(width))
+				}
+				fmt.Fprintf(&buf, "%s\n", line)
+			}
+		} else {
+			fmt.Fprintf(&buf, "%s\n", desc.Detail)
 		}
-		fmt.Fprintf(&buf, "%s\n", detail)
 	}
 
 	return buf.String()

command/format/diagnostic_test.go

@@ -167,3 +167,35 @@ Error: Some error
 		t.Fatalf("unexpected output: got:\n%s\nwant\n%s\n", output, expected)
 	}
 }
+
+func TestDiagnostic_wrapDetailIncludingCommand(t *testing.T) {
+	var diags tfdiags.Diagnostics
+
+	diags = diags.Append(&hcl.Diagnostic{
+		Severity: hcl.DiagError,
+		Summary:  "Everything went wrong",
+		Detail:   "This is a very long sentence about whatever went wrong which is supposed to wrap onto multiple lines. Thank-you very much for listening.\n\nTo fix this, run this very long command:\n  terraform read-my-mind -please -thanks -but-do-not-wrap-this-line-because-it-is-prefixed-with-spaces\n\nHere is a coda which is also long enough to wrap and so it should eventually make it onto multiple lines. THE END",
+	})
+	color := &colorstring.Colorize{
+		Colors:  colorstring.DefaultColors,
+		Reset:   true,
+		Disable: true,
+	}
+	expected := `
+Error: Everything went wrong
+
+This is a very long sentence about whatever went wrong which is supposed to
+wrap onto multiple lines. Thank-you very much for listening.
+
+To fix this, run this very long command:
+  terraform read-my-mind -please -thanks -but-do-not-wrap-this-line-because-it-is-prefixed-with-spaces
+
+Here is a coda which is also long enough to wrap and so it should eventually
+make it onto multiple lines. THE END
+`
+	output := Diagnostic(diags[0], nil, color, 76)
+
+	if output != expected {
+		t.Fatalf("unexpected output: got:\n%s\nwant\n%s\n", output, expected)
+	}
+}