t7610-mergetool: do not place pipelines headed by `yes` in subshells

t7610-mergetool: do not place pipelines headed by `yes` in subshells

Subshells for pipelines are not required. This can save a number of
processes (if the shell does not optimize it away anyway).

The patch was generated with the command

   sed -i 's/( *\(yes.*[^ ]\) *) *\&\&/\1 \&\&/' t7610-mergetool.sh

with a manual fixup of the case having no && at the end.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>