Update .gitignore.
master: always initialize the default cgiurl_graph value.

The cgiurl_graph parameter in $config is not set by defaule the same
way as the other values in Config.pm, as it may be calculated from
$config->{cgiurl}, if present.

Instead of initializing only when the revision does not match, do so
every time after loading the datafile section, as to avoid the
following warning:

    Use of uninitialized value in concatenation (.) or string at /usr/share/perl5/vendor_perl/Munin/Master/HTMLConfig.pm line 486.
master: simplify logic for mapping images to their path.

This is equivalent but we split the logic of deciding where the path
is going to be from setting it, using a single map() call.
master: provide a default to graph_strategy.

This allows $config->{graph_strategy} to be usable instead of
requiring munin_get.
master: avoid repeated tests on graph_strategy.
master: remove unused variable.