run-command(win32): resolve the path to the Unix shell early

run-command(win32): resolve the path to the Unix shell early

In 776297548e (Do not use SHELL_PATH from build system in
prepare_shell_cmd on Windows, 2012-04-17), the hard-coded path to the
Unix shell was replaced by passing `sh` instead when executing Unix
shell scripts in Git.

This was done because the hard-coded path to the Unix shell is incorrect
on Windows because it not only is a Unix-style absolute path instead of
a Windows one, but Git uses the runtime prefix feature on Windows, i.e.
the correct path cannot be hard-coded.

Naturally, the `sh` argument will be resolved to the full path of said
executable eventually.

To help fixing the bug where `git var GIT_SHELL_PATH` currently does not
reflect that logic, but shows that incorrect hard-coded Unix-style
absolute path, let's resolve the full path to the `sh` executable early
in the `git_shell_path()` function so that we can use it in `git var`,
too, and be sure that the output is equivalent to what `run_command()`
does when it is asked to execute a command-line using a Unix shell.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>