svnmailer: add log substitution for subject templates

Add first line of commit log as a substitution variable for
*_subject_template, E.G. add %(log)s to use.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Index: svnmailer-1.0.8/src/lib/svnmailer/notifier/_mail.py
===================================================================
--- svnmailer-1.0.8.orig/src/lib/svnmailer/notifier/_mail.py	2008-05-27 14:56:27.000000000 +0200
+++ svnmailer-1.0.8/src/lib/svnmailer/notifier/_mail.py	2008-05-27 15:50:29.000000000 +0200
@@ -314,6 +314,7 @@
             'part'    : countprefix,
             'files'   : self._getPrefixedFiles(changeset),
             'dirs'    : self._getPrefixedDirectories(changeset),
+            'log'     : self.getLog().split('\n', 1)[0],
         }
 
         # We may try twice, first with files/dirs = files