The build procedure learned to optionally use symbolic links
(instead of hardlinks and copies) to install "git-foo" for built-in
commands, whose binaries are all identical.

* ab/install-symlinks:
  Makefile: optionally symlink libexec/git-core binaries to bin/git
  Makefile: add a gitexecdir_relative variable
  Makefile: fix broken bindir_relative variable