Makefile: stop using `test -o` when unlinking duplicate executables

Makefile: stop using `test -o` when unlinking duplicate executables

When building executables we may end up with both `foo` and `foo.exe` in
the project's root directory. This can cause issues on Cygwin, which is
why we unlink the `foo` binary (see 6fc301bbf68 (Makefile: remove $foo
when $foo.exe is built/installed., 2007-01-10)). This step is skipped if
either:

    - `foo` is a directory, which can happen when building Git on
      Windows via MSVC (see ade2ca0ca9f (Do not try to remove
      directories when removing old links, 2009-10-27)).

    - `foo` is a hardlink to `foo.exe`, which can happen on Cygwin (see
      0d768f7c8f1 (Makefile: building git in cygwin 1.7.0, 2008-08-15)).

These two conditions are currently chained together via `test -o`, which
is discouraged by our code style guide. Convert the recipe to instead
use an `if` statement with `&&`'d conditions, which both matches our
style guide and is easier to ready.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>