mergetool: no longer need to save standard input

mergetool: no longer need to save standard input

Earlier code wanted to run merge_file and prompt_after_failed_merge
both of which wanted to read from the standard input of the entire
script inside a while loop, which read from a pipe, and in order to
do so, it redirected the original standard input to another file
descriptor. We no longer need to do so after the previous change.

Signed-off-by: Junio C Hamano <gitster@pobox.com>