Makefile: link builtins residing in bin directory to main git binary too

Makefile: link builtins residing in bin directory to main git binary too

To conserve space/improve file caching we try to make hard or symbolic links
from each builtin program to the main git executable rather than having
each be a complete duplicate copy of it.  We weren't doing this for the
builtin programs residing in the bin directory though.  So, let's do so.

Signed-off-by: Brandon Casey <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>