gdb sometimes writes warnings to stdout, which we don't need/want, and
for some reason --batch-silent isn't enough to hide. So in this commit
we write them to a log file, which is probably better for debugging
anyway. To see an illustrative test failure before this change, run
% make
% touch notmuch-count.c
% cd test && ./T060-count.sh
(cherry picked from commit
f45fa5bdd397d52473f7092f7ae3e2ffb9b7aee5)
cat <<EOF > count-files.gdb
set breakpoint pending on
+set logging file count-files-gdb.log
+set logging on
break count_files
commands
shell cp /dev/null ${MAIL_DIR}/.notmuch/xapian/postlist.${db_ending}
gen_insert_msg
cat <<EOF > index-file-$code.gdb
set breakpoint pending on
+set logging file index-file-$code.log
+set logging on
break notmuch_database_add_message
commands
return NOTMUCH_STATUS_$code