[PATCH] issue #4527: notify start of exporting external

Please find attached a patch which provides the missing notification at the start of exporting externals. Since I've never before worked on subversion code, please review it especially carefully :) I apologize for possible blunt errors in advance.

This patch only fixes the missing notification of starting to export externals. It does not address the second part of issue #4527, which is the premature reset of the in_external flag in the case of nested externals, which also affects checkouts. To me it seems that the best option for fixing that issue would be to turn the bool flag into a counter which keeps track of the "stack deptch" of externals, but I am not totally sure about how this affects the ABI and whether there might be better ways of fixing this. Any comments/ideas? Since this is not really the same issue but more of a closely related issue, I can turn this into a second issue if this is preferred.

[[[
(Partially) Fix Issue #4527: Notify start of exporting external

* subversion/libsvn_client/externals.c
   (svn_client__export_externals): Send svn_wc_notify_update_external notification before handling external.
]]]

Cheers,
Michael