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>