Do not try to remove directories when removing old links

Do not try to remove directories when removing old links

When building Git with MSVC on Windows, directories named after the Git alias
are created for the output files, e.g. there is a "git-merge-index" directory
next to the "git-merge-index.exe" executable in the build root. Previously,
"make all" just checked if "git-merge-index" and "git-merge-index.exe" are the
same file, and if not, tried to remove "git-merge-index". This fails in the
case of "git-merge-index" being a directory, which is why this is checked now.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>