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>