Fix relative built-in paths to be relative to the command invocation

Fix relative built-in paths to be relative to the command invocation

$(gitexecdir) (as defined in the Makefile) has gained another path
component, but the relative paths in the MINGW section of the Makefile,
which are interpreted relative to it, do not account for it.

Instead of adding another ../ in front of the path, we change the code that
constructs the absolute paths to do it relative to the command's directory,
which is essentially $(bindir). We do it this way because we will also
allow a relative $(gitexecdir) later.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>