Windows: set gitexecdir = $(bindir)
The "dash-less" change aims to remove git commands from $PATH. It does so
by defining a GIT_EXEC_PATH that is different from $(bindir). On Windows
we want a relocatable installation of the git tool, so we cannot use an
absolute GIT_EXEC_PATH. Therefore, the implementation of
builtin_exec_path() on Windows derives the exec-path from the command
invocation, and disregards GIT_EXEC_PATH. But this broke when
$(gitexecdir) became different from $(bindir), so we restore the earlier
behavior here.
This counteracts the aims of the "dash-less" change on Windows, but better
this way than having no working git at all.
Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>