shortlog: stop setting pp.print_email_subject

When shortlog processes a commit using its internal traversal, it may
pretty-print the subject line for the summary view. When we do so, we
set the "print_email_subject" flag in the pretty-print context. But this
flag does nothing! Since we are using CMIT_FMT_USERFORMAT, we skip most
of the usual formatting code entirely.

This flag is there due to commit 6d167fd7cc (pretty: use
fmt_output_email_subject(), 2017-03-01). But that just switched us away
from setting an empty "subject" header field, which was similarly
useless. That was added by dd2e794a21 (Refactor pretty_print_commit
arguments into a struct, 2009-10-19). Before using the struct, we had to
pass _something_ as the argument, so we passed the empty string (a NULL
would have worked equally well).

So this setting has never done anything, and we can drop the line. That
shortens the code, but more importantly, makes it easier to reason about
and refactor the other users of this flag.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/shortlog.c

@@ -245,7 +245,6 @@ void shortlog_add_commit(struct shortlog *log, struct commit *commit)
 
 	ctx.fmt = CMIT_FMT_USERFORMAT;
 	ctx.abbrev = log->abbrev;
-	ctx.print_email_subject = 1;
 	ctx.date_mode = log->date_mode;
 	ctx.output_encoding = get_log_output_encoding();