$X is appended to binary names for Windows builds (ie. git.exe).
Pollution from the environment can inadvertently trigger this behaviour,
resulting in 'git' turning into 'gitwhatever' without warning.
Signed-off-by: Michael Palimaka <kensington@astralcloak.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>