Windows: Make sure argv[0] has a path

Windows: Make sure argv[0] has a path

Since the exec-path on Windows is derived from the program invocation path,
we must ensure that argv[0] always has a path. Unfortunately, if a program
is invoked from CMD, argv[0] has no path. But on the other hand, the
C runtime offers a global variable, _pgmptr, that always has the full path
to the program. We hook into main() with a preprocessor macro, where we
replace argv[0].

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>