ui: add environ property to access os.environ or wsgirequest.environ
The property returns os.environ by default, and is propagated by ui.copy.
During hgweb processing, ui.environ is set to the proper WSGI-request
environment, as contained in wsgirequest.environ. For CGI, this is the
same as os.environ.
The property is meant to be read-only, as with os.environ (generally).
When Mercurial accepts more than one revision, they may be specified
individually, or provided as a topologically continuous range,
separated by the ":" character.
The syntax of range notation is [BEGIN]:[END], where BEGIN and END are
revision identifiers. Both BEGIN and END are optional. If BEGIN is not
specified, it defaults to revision number 0. If END is not specified,
it defaults to the tip. The range ":" thus means "all revisions".
If BEGIN is greater than END, revisions are treated in reverse order.
A range acts as a closed interval. This means that a range of 3:5
gives 3, 4 and 5. Similarly, a range of 9:6 gives 9, 8, 7, and 6.