Merge branch 'ab/install-symlinks'

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