git-send-email-reply: Append subject

git-send-email-reply: Append subject

I keep copy-pasting the addresses provided,
I keep writing my plaintext reply in a file,
and I keep forgetting to add a subject
(because I am "just" writing a plaintext file)

Teach `git-send-email-reply` to append a `--subject` line.

[ew: avoid URI-encoded subject on command-line, adjust t/reply.t]

Signed-off-by: Stavros Ntentos <133706+stdedos@users.noreply.github.com>