send-email: programmatically generate bash completions

send-email: programmatically generate bash completions

"git send-email --git-completion-helper" only prints "format-patch"
flags. Make it print "send-email" flags as well, extracting them
programmatically from its three existing "GetOptions".

Introduce a "uniq" subroutine, otherwise --cc-cover, --to-cover and
other flags would show up twice. In addition, deduplicate flags common
to both "send-email" and "format-patch", like --from.

Remove extraneous flags: --h and --git-completion-helper.

Add trailing "=" to options that expect an argument, inline with
the format-patch implementation.

Add a completion test for "send-email --validate", a send-email flag.

Signed-off-by: Thiago Perrotta <tbperrotta@gmail.com>
Based-on-patch-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>