Otherwise the build fails with the perplexing error
make: *** No rule to make target 'cmd-list.made', needed by 'doc.dep'. Stop.
make: Leaving directory '/tmp/nix-build-git-2.16.3.drv-0/git-2.16.3/Documentation'
on NixOS (but not on Debian, where it succeeds, presumably since it picks up the
system perl).