t2300: run git-sh-setup in an environment that better mimics the real life

t2300: run git-sh-setup in an environment that better mimics the real life

When we run scripted Porcelains, "git" potty has set up the $PATH by
prepending $GIT_EXEC_PATH, the path given by "git --exec-path=$there
$cmd", etc. already.  Because of this, scripted Porcelains can
dot-source shell script library like git-sh-setup with simple dot
without specifying any path.

t2300 however dot-sources git-sh-setup without adjusting $PATH like
the real "git" potty does.  This has not been a problem so far, but
once git-sh-setup wants to rely on the $PATH adjustment, just like
any scripted Porcelains already do, it would become one.  It cannot
for example dot-source another shell library without specifying the
full path to it by prefixing $(git --exec-path).

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