bin-wrappers: append `.exe` to target paths if necessary

bin-wrappers: append `.exe` to target paths if necessary

When compiling with Visual Studio, the projects' names are identical to
the executables modulo the extensions. Read: there will exist both a
directory called `git` as well as an executable called `git.exe` in the
end. Which means that the bin-wrappers *need* to target the `.exe` files
lest they try to execute directories.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>