Prepare execv_git_cmd() for removal of builtins from the filesystem

Prepare execv_git_cmd() for removal of builtins from the filesystem

Currently, execv_git_cmd() always try running the dashed form, which
means we cannot easily remove the git-foo hardlinks for built-in
commands.  This updates the function to always exec "git foo" form, and
makes sure "git" potty does not infinitely recurse to itself.

Signed-off-by: Junio C Hamano <gitster@pobox.com>