t3600: stop losing return codes of git commands

t3600: stop losing return codes of git commands

When a command is in a non-assignment command substitution, the return
code will be lost in favour of the surrounding command's. As a result,
if a git command fails, we won't know about it. Rewrite instances of
this so that git commands are either run in an assignment-only command
substitution so that their return codes aren't lost.

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