stop installing git-tar-tree link

When the built-in "git tar-tree" command (a thin wrapper around "git
archive") was removed in 925ceccf (tar-tree: remove deprecated
command, 2013-11-10), the build continued to install a non-functioning
git-tar-tree command in gitexecdir by mistake:

	$ PATH=$(git --exec-path):$PATH
	$ git-tar-tree -h
	fatal: cannot handle tar-tree internally

The list of links in gitexecdir is populated from BUILTIN_OBJS, which
includes builtin/tar-tree.o to implement "git get-tar-commit-id".
Rename the get-tar-commit-id source file to builtin/get-tar-commit-id.c
to reflect its purpose and fix 'make install'.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>