subtree: fix the GIT_EXEC_PATH sanity check to work on Windows

In 22d550749361 (subtree: don't fuss with PATH, 2021-04-27), `git subtree` was broken thoroughly on Windows. The reason is that it assumes Unix semantics, where `PATH` is colon-separated, and it assumes that `$GIT_EXEC_PATH:` is a verbatim prefix of `$PATH`. Neither are true, the latter in particular because `GIT_EXEC_PATH` is a Windows-style path, while `PATH` is a Unix-style path list. Let's make extra certain that `$GIT_EXEC_PATH` and the first component of `$PATH` refer to different entities before erroring out. We do that by using the `test <path1> -ef <path2>` command that verifies that the inode of `<path1>` and of `<path2>` is the same. Sadly, this construct is non-portable, according to https://pubs.opengroup.org/onlinepubs/009695399/utilities/test.html. However, it does not matter in practice because we still first look whether `$GIT_EXEC_PREFIX` is string-identical to the first component of `$PATH`. This will give us the expected result everywhere but in Git for Windows, and Git for Windows' own Bash _does_ handle the `-ef` operator. Just in case that we _do_ need to show the error message _and_ are running in a shell that lacks support for `-ef`, we simply suppress the error output for that part. This fixes https://github.com/git-for-windows/git/issues/3260 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>