send-email: don't cc *-by lines with '-' prefix

send-email: don't cc *-by lines with '-' prefix

Since commit ef0cc1df90f6b ("send-email: also pick up cc addresses from
-by trailers") in git version 2.20, git send-email adds to cc list
addresses from all *-by lines. As a side effect a line with
'-Signed-off-by' is now also added to cc. This makes send-email pick
lines from patches that remove patch files from the git repo. This is
common in the Buildroot project that often removes (and adds) patch
files that have 'Signed-off-by' in their patch description part.

Consider only *-by lines that start with [a-z] (case insensitive) to
avoid unrelated addresses in cc.

Cc: Joe Perches <joe@perches.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>