update-doc-branches: add command to update man and html

update-doc-branches: add command to update man and html

Create a new command to update the man and html branches, and move the
related code there from the Makefile.

Update the branches based on the current (clean) tree, rather than
consulting the git origin, and rely on ls-files rather than globbing
so that the file lists will always be correct -- we'll immediately
notice deletions, avoid picking up stray files in the directory, etc.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>