When '--compose' is used, git send-email will use the From, Subject, and
In-Reply-To headers specified in the message. If the body of the message
(what you type after the headers and a blank line) only contains blank
-(or GIT: prefixed) lines the summary won't be sent, but From, Subject,
+(or Git: prefixed) lines the summary won't be sent, but From, Subject,
and In-Reply-To headers will be used unless they are removed.
+
Missing From or In-Reply-To headers will be prompted for.