t7528: fix failure under csh

t7528: fix failure under csh

Explicitly set sh mode for ssh-agent (ssh-agent -s) to prevent
failure when user's login shell is csh-like.  The failure is
caused by propagation of the $SHELL value from the user's original
shell despite the test and test harness explictly using sh, which
makes ssh-agent emit initialization code for the wrong shell:

> cd t
> echo $SHELL
/bin/tcsh
> ./t7528-signed-commit-ssh.sh --verbose --debug
[...]
expecting success of 7528.2 'sign commits using literal public keys with ssh-agent':
[...]
./t7528-signed-commit-ssh.sh: 1: eval: setenv: not found
./t7528-signed-commit-ssh.sh: 1: eval: setenv: not found
[...]

Signed-off-by: Kenneth Lorber <keni@his.com>
Acked-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>