exec_cmd.c: use find_last_dir_sep() for code simplification

exec_cmd.c: use find_last_dir_sep() for code simplification

We are trying to extract dirname from argv0 in the git_extract_argv0_path().
But in the same time, the <git-compat-util.h> provides find_last_dir_sep()
to get dirname from a given path.  Let's use it instead of loop for the code
simplification.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>