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>

Makefile

@@ -468,8 +468,7 @@ ARFLAGS = rcs
 # This can help installing the suite in a relocatable way.
 
 prefix = $(HOME)
-bindir_relative = bin
-bindir = $(prefix)/$(bindir_relative)
+bindir = $(prefix)/bin
 mandir = $(prefix)/share/man
 infodir = $(prefix)/share/info
 gitexecdir = libexec/git-core
@@ -486,6 +485,7 @@ lib = lib
 # DESTDIR =
 pathsep = :
 
+bindir_relative = $(patsubst $(prefix)/%,%,$(bindir))
 mandir_relative = $(patsubst $(prefix)/%,%,$(mandir))
 infodir_relative = $(patsubst $(prefix)/%,%,$(infodir))
 htmldir_relative = $(patsubst $(prefix)/%,%,$(htmldir))