Makefile: install 'git' in execdir

Makefile: install 'git' in execdir

When a git command executes a subcommand, it uses the "git
foo" form, which relies on finding "git" in the PATH.
Normally this should not be a problem, since the same "git"
that was used to invoke git in the first place will be
found.  And if somebody invokes a "git" outside of the PATH
(e.g., by giving its absolute path), this case is already
covered: we put that absolute path onto the front of PATH.

However, if one is using "sudo", then sudo will execute the
"git" from the PATH, but pass along a restricted PATH that
may not contain the original "git" directory. In this case,
executing a subcommand will fail.

To solve this, we put the "git" wrapper itself into the
execdir; this directory is prepended to the PATH when git
starts, so the wrapper will always be found.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>