git-gui: _which, only add .exe suffix if not present

git-gui: _which, only add .exe suffix if not present

The _which function finds executables on $PATH, and adds .exe on Windows
unless -script was given. However, win32.tcl executes "wscript.exe"
and "cscript.exe", both of which fail as _which adds .exe to both. This
is already fixed in git-gui released by Git for Windows. Do so here.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>