Add support for using EDITOR to construct commit messages.

message: |-
  Add support for using EDITOR to construct commit messages.

  I feel like I can breathe again! The code is more functional than aesthetic, but
  it works damnit!
change_hash: AP+JESJn2+CP1FCDvwJqaXUM354rpoK5oU6Me4uByxaf
credentials:
- type: pgp_signature
  pub_key_id: 95C46FA6A41148AC
  body: 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
  account: mediocregopher

cmd/dehub/main.go

@@ -2,12 +2,15 @@ package main
 
 import (
 	"bufio"
+	"bytes"
 	"dehub"
 	"errors"
 	"flag"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
+	"os/exec"
 	"strings"
 
 	"gopkg.in/src-d/go-git.v4/plumbing"
@@ -20,6 +23,63 @@ func exitErr(err error) {
 	os.Exit(1)
 }
 
+func tmpFileMsg() (string, error) {
+	editor := os.Getenv("EDITOR")
+	if editor == "" {
+		return "", errors.New("EDITOR not set, please set it or use -msg in order to create your commit message")
+	} else if _, err := os.Stat(editor); err != nil {
+		return "", fmt.Errorf("could not stat EDITOR %q: %w", editor, err)
+	}
+
+	tmpf, err := ioutil.TempFile("", "")
+	if err != nil {
+		return "", fmt.Errorf("could not open temp file: %w", err)
+	}
+	tmpfName := tmpf.Name()
+	defer os.Remove(tmpfName)
+
+	tmpBody := bytes.NewBufferString(`
+
+# Please enter the commit message for your changes. Lines starting
+# with '#' will be ignored, and an empty message aborts the commit.`)
+
+	_, err = io.Copy(tmpf, tmpBody)
+	tmpf.Close()
+	if err != nil {
+		return "", fmt.Errorf("could not write helper message to temp file %q: %w", tmpfName, err)
+	}
+
+	cmd := exec.Command(editor, tmpfName)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("error running '%s %q': %w", editor, tmpfName, err)
+	}
+
+	body, err := ioutil.ReadFile(tmpfName)
+	if err != nil {
+		return "", fmt.Errorf("error retrieving message body from %q: %w", tmpfName, err)
+	}
+
+	bodyFiltered := new(bytes.Buffer)
+	bodyBR := bufio.NewReader(bytes.NewBuffer(body))
+	for {
+		line, err := bodyBR.ReadString('\n')
+		if errors.Is(err, io.EOF) {
+			break
+		} else if err != nil {
+			return "", fmt.Errorf("error reading from buffered body: %w", err)
+		}
+
+		if !strings.HasPrefix(strings.TrimSpace(line), "#") {
+			bodyFiltered.WriteString(line)
+		}
+	}
+
+	return strings.TrimSpace(bodyFiltered.String()), nil
+}
+
 type subCmdCtx struct {
 	repo func() *dehub.Repo
 	flag *flag.FlagSet
@@ -46,9 +106,19 @@ var subCmds = []subCmd{
 			accountID := sctx.flag.String("account-id", "", "Account to sign commit as")
 			sctx.flagParse()
 
-			if *msg == "" || *accountID == "" {
+			if *accountID == "" {
 				flag.PrintDefaults()
-				return errors.New("-msg and -account-id are both required")
+				return errors.New("-account-id is required")
+			}
+
+			if *msg == "" {
+				var err error
+				if *msg, err = tmpFileMsg(); err != nil {
+					return fmt.Errorf("error collecting commit message from user: %w", err)
+
+				} else if *msg == "" {
+					return errors.New("empty commit message, not doing anything")
+				}
 			}
 
 			cfg, err := sctx.repo().LoadConfig()