pretty: drop print_email_subject flag

With one exception, the print_email_subject flag is set if and only if the commit format is email based: - in make_cover_letter() we set it along with CMIT_FMT_EMAIL explicitly - in show_log(), we set it if cmit_fmt_is_mail() is true. That covers format-patch as well as "git log --format=email" (or mboxrd). The one exception is "rev-list --format=email", which somewhat nonsensically prints the author and date as email headers, but no subject, like: $ git rev-list --format=email HEAD commit 64fc4c2cdd4db2645eaabb47aa4bac820b03cdba From: Jeff King <peff@peff.net> Date: Tue, 19 Mar 2024 19:39:26 -0400 this is the subject this is the body It's doubtful that this is a useful format at all (the "commit" lines replace the "From" lines that would make it work as an actual mbox). But I think that printing the subject as a header (like this patch does) is the least surprising thing to do. So let's drop this field, making the code a little simpler and easier to reason about. Note that we do need to set the "rev" field of the pretty_print_context in rev-list, since that is used to check for subject_prefix, etc. It's not possible to set those fields via rev-list, so we'll always just print "Subject: ". But unless we pass in our rev_info, fmt_output_email_subject() would segfault trying to figure it out. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>