Makefile: installing git in cygwin 1.7.0

Makefile: installing git in cygwin 1.7.0

On platforms with $X, make removes any leftover scripts 'a' from
earlier builds if a new binary 'a.exe' is now built.  However, on
cygwin 1.7.0, 'git' and 'git.exe' now consistently name the same file.
Test for file equality before attempting a remove, in order to avoid
nuking just-built binaries.

This repeats commit 0d768f7 for the installation destdir.

Signed-off-by: Eric Blake <ebb9@byu.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>