Set GIT_DIR instead of changing directory in find_git_or_skip() in test.pl

Set GIT_DIR instead of changing directory in find_git_or_skip() in test.pl

Return the directory of the source tree, so that tests can change directory
to it if necessary. Modify t/porting/cmp_version.t to change directory.

This gets t/porting/utils.t passing again under -Dmksymlinks, without breaking
t/porting/cmp_version.t. The former needs to run in the build tree as it
inspects various build products. The latter needs to run in the (real)
checkout tree, else it thinks that every single file has changed (into a
symlink).