PATHEXT lists file extensions with the ".". In the fallback given
in _get_exe_extensions, the other extensions had this, but ".COM"
was listed without the ".". This fixes that.

This is very minor because _get_exe_extensions is nonpublic and not
currently used on native Windows, which is the platform where the
PATHEXT fallback code would be used.

Specifically, _get_exe_extensions is called only in py_where, which
while named with no leading underscore is nonpublic do not being
(and never having been) listed in __all__. As its docstring states,
it is an implementation detail of is_cygwin_git and not intended
for any other use. More specifically, is_cygwin_git currently
immediately returns False on *native* Windows (even if the git
executable GitPython is using is a Cygwin git executable). Only on
Cygwin, or other systems that are not native Windows, does it try
to check the git executable (by calling its _is_cygwin_git helper,
which uses py_where).