configitems: move the default values to the registrar decorator
The exthelper refactoring of configitems stopped registering dynamicdefault for
everything, which in turn causes a lot of warnings about mismatched defaults.
There are a few stragglers- I didn't fix topic-mode because I wasn't sure what
the default should be, and the alias for odiff was explicitly given a default
for compatibility.
. $TESTDIR/testlib/pythonpath.sh
cat >> $HGRCPATH << EOF
[extensions]
docgraph=
EOF