subtree: fix the GIT_EXEC_PATH sanity check to work on Windows

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>