send-email: teach git send-email option to translate aliases

send-email: teach git send-email option to translate aliases

git send-email has support for converting shorthand alias names to
canonical email addresses via the alias file. It supports a wide variety
of alias file formats based on popular email program file formats.

Other programs, such as b4, would like the ability to convert aliases in
the same way as git send-email without needing to re-implement the logic
for understanding the many file formats.

Teach git send-email a new option, --translate-aliases, which will
enable this functionality. Similar to --dump-aliases, this option works
like a new mode of operation for git send-email.

When run with --translate-aliases, git send-email reads from standard
input and converts any provided alias into its canonical name and email
according to the alias file. Each expanded name and address is printed
to standard output, one per line.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>