Windows: Wrap execve so that shell scripts can be invoked.

Windows: Wrap execve so that shell scripts can be invoked.

When an external git command is invoked, it can be a Bourne shell script.
This patch looks into the command file to see whether it is one.
In this case, the command line is rearranged to invoke the shell
with the proper arguments.

With this change, scripted git commands work. Command line arguments
to those scripts cannot be complex (contain spaces or double-quotes), yet.

Signed-off-by: Johannes Sixt <redacted>