Write svn_revision when getting the revision from git

Originally committed to SVN as r6297.

aegisub/configure.in

@@ -163,6 +163,7 @@ AS_IF([test -d "$srcdir/.svn"], [
   AC_MSG_RESULT([$SVN_REVISION from "$srcdir/svn_revision"])
 ], [test -d "$srcdir/../.git"], [
   SVN_REVISION=[`git log | grep git-svn-id -m 1 | sed -E 's/.*@([0-9]+) .*/\1/'`]
+  `echo $SVN_REVISION > $srcdir/svn_revision`
   AC_MSG_RESULT([$SVN_REVISION from git])
 ], [
   AC_MSG_RESULT([not found])