Makefile: fix shell quoting
Makefile records paths to a few programs in GIT-BUILD-OPTIONS file. These
paths need to be quoted twice: once to protect specials from the shell
that runs the generated GIT-BUILD-OPTIONS file, and again to protect them
(and the first level of quoting itself) from the shell that runs the
"echo" inside the Makefile.
You can test this by trying:
$ ln -s /bin/tar "$HOME/Tes' program/tar"
$ make TAR="$HOME/Tes' program/tar" test
Signed-off-by: Junio C Hamano <gitster@pobox.com>