Makefile: fix broken bindir_relative variable

Makefile: fix broken bindir_relative variable

Change the bindir_relative variable to work like the other *_relative
variables, which are computed as a function of the absolute
path. Before this change, supplying e.g. bindir=/tmp/git/binaries to
the Makefile would yield a bindir_relative of just "bin", as opposed
to "binaries".

This logic was originally added back in 026fa0d5ad ("Move computation
of absolute paths from Makefile to runtime (in preparation for
RUNTIME_PREFIX)", 2009-01-18), then later in 971f85388f ("Makefile:
make mandir, htmldir and infodir absolute", 2013-02-24) when
more *_relative variables were added those new variables didn't have
this bug, but bindir_relative was never fixed.

There is a small change in behavior here, which is that setting
bindir_relative as an argument to the Makefile won't work anymore, I
think that's fine, since this was always intended as an internal
variable (e.g. INSTALL documents bindir=*, not bindir_relative=*).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>