t5703: stop losing return codes of git commands

t5703: stop losing return codes of git commands

Currently, there are two ways where the return codes of git commands are
lost. The first way is when a command is in the upstream of a pipe. In a
pipe, only the return code of the last command is used. Thus, all other
commands will have their return codes masked. Rewrite pipes so that
there are no git commands upstream.

The other way is when a command is in a non-assignment command
substitution. The return code will be lost in favour of the surrounding
command's. Rewrite instances of this such that git commands are in an
assignment-only command substitution.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>