Add support for export when building

[pkruithof]

As is stands now, the export task actually does the following:

For git: git clone
For svn: svn export

These are not really equivalents: a svn export is a distribution of the source code, without the version control history etc. While the git clone command actually creates a local repository.

The git equivalent seems to be archive. The reason I'm asking is because I wanted to use .gitattributes to exclude certain paths from exporting (to save time), but that does not work with clone.

Not sure if the default action should be changed (seems like a BC break), or if it should be configurable. But it would be nice if this was fixed to be similar across VCS systems.