Sadly I cannot use this myself ;-).
Two points and a half.
(1) DESTDIR traditionally is always used to denote the virtual
root directory for fake install, typically later to be
copied over elsewhere or tarred up (e.g. "install git.1
$(DESTDIR)/usr/share/man/man1/git.1"); I'd suggest to make
install-doc-quick.sh script take two parameters, the branch
and the destination directory.(2) "git-clone" makes my html and man branches tracked with
remotes/origin/{html,man} by default; you would want to make
the local tracking branch 'man' you have hardcoded in the
Makefile overridable and make it default to "origin/man".(2.5) "chmod +x install-doc-quick.sh", even if you say "sh
./..." in your Makefile.