Replace DOS linefeeds with Unix line feeds when loading a commit message

I have seen some commit messages that contain CRLF instead of just LF; I'm not
sure if these were created by a broken git client, but they exist, so we need to
deal with them. Editing them when rewording a commit sort of works, but is a
little strange; the \r characters are invisble, so you need an extra arrow key
press to skip over them.

In the next commit we are going to add more logic related to line breaks, and it
is getting confused by the \r, so it is becoming more important to fix this. The
easiest fix is to normalize the line endings right after loading.

pkg/commands/git_commands/commit.go

@@ -142,7 +142,7 @@ func (self *CommitCommands) GetCommitMessage(commitSha string) (string, error) {
 		ToArgv()
 
 	message, err := self.cmd.New(cmdArgs).DontLog().RunWithOutput()
-	return strings.TrimSpace(message), err
+	return strings.ReplaceAll(strings.TrimSpace(message), "\r\n", "\n"), err
 }
 
 func (self *CommitCommands) GetCommitSubject(commitSha string) (string, error) {