Fix typeerror when specifying both --rebase and --pull

coal, paper: omit unneeded #l1 anchor in links

localrepo: corrected outdated comment

localrepo: fixed typos in comments

help: document nonempty template filter.

webcommands: move nonempty logic from JavaScript to Python This avoids hard-coding the '(none)' string in the JavaScript for each style and avoids the {nonexisting|nonempty} hack which only works since the template system don't complain about undefined variables.

hgweb: use "nonempty" filter in templates for the changelog message At least subversion allows empty commit logs which, after conversion, show up as empty fields in hgweb and don't allow selecting the changeset. Using "nonempty" as filter, "(none)" is used to get clickable links for empty commit messages.

templatefilters: add "nonempty" template filter It ensures that at least "(none)" is returned in case the argument passed is None or ''. This is primarily useful to render empty changelog messages for hgweb but may be useful for others, too.

keepalive: fix 4f13ed6ee544, reintroduce unredirected_hdrs The previous fix dropped unredirected_hdrs which contain authentication headers. Removing them break POST request requiring authentication (like unbundle calls to bitbucket.org).

pure/osutil: add copyright and license header