A subversion repository can contain externals, references to other
projects that also use subversion. This is very similar to git
submodules. When checking out a repository, svn will automatically
check these external repositories too. However, if the externals are
not pinned to a specific revision, svn will fetch the latest sources
for the external, leading to non reproducibility of the sources' hash.

This patch adds a fetch-externals? field to svn-reference and
svn-multi-reference, defaulting to #t for preserving the old behavior.
Setting it to #f will prevent fetching any external from the project.
This is useful for josm for instance.