send-email: Use To: headers in patch files

send-email: Use To: headers in patch files

It's a minor annoyance when you take the painstaking time to setup To:
headers for each patch in a large series, and then go out to send the
series with git-send-email and watch git ignore the To: headers in the
patch files.

Therefore, always add To: headers from a patch file to the To: headers
for that message. Keep the prompt for the blanket To: header so as to
not break scripts (and user expectations). This means even if a patch
has a To: header, git will prompt for the To: address. Otherwise, we'll
need to introduce interface breakage to either request the header for
each patch missing a To: header or default the header to whatever To:
address is found first (be it in a patch or from user input). Both of
these options don't seem very obvious/useful.

Reported-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Tested-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>