system_path(): simplify using strip_path_suffix(), and add suffix "git"

system_path(): simplify using strip_path_suffix(), and add suffix "git"

At least for the author of this patch, the logic in system_path() was
too hard to understand.  Using the function strip_path_suffix() documents
the idea of the code better.

The real change is to add the suffix "git", so that a runtime prefix will
be computed correctly even when the executable was called in /git/ as is
the case in msysGit (Windows insists to search the current directory
before the PATH when looking for an executable).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>