git-svn: control destruction order to avoid segfault

It seems necessary to control destruction ordering to avoid a
segfault with SVN 1.9.5 when using "git svn branch".  I've also
reported the problem against libsvn-perl to Debian [Bug #888791
  
An error occurred
 
],
but releasing the SVN::Client instance can be beneficial anyways to
save memory.

ref: https://bugs.debian.org/888791
Tested-by: Todd Zullinger <tmz@pobox.com>
Reported-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-svn.perl

@@ -1199,6 +1199,11 @@ sub cmd_branch {
 	$ctx->copy($src, $rev, $dst)
 		unless $_dry_run;
 
+	# Release resources held by ctx before creating another SVN::Ra
+	# so destruction is orderly.  This seems necessary with SVN 1.9.5
+	# to avoid segfaults.
+	$ctx = undef;
+
 	$gs->fetch_all;
 }