Add test to show dulwich fault on invalid timezone data

A recent real world occurrence - user hand edited the timezone field in
an hg export to provide a unique value (from prior export). Hg imported
the export okay, but dulwich threw an exception.

This test shows the fault.
1 files changed, 32 insertions(+), 0 deletions(-)

A => tests/test-timezone.t
A => tests/test-timezone.t +32 -0
@@ 0,0 1,32 @@ 
+This test shows how dulwich fails to convert a commit accepted by hg.
+
+In the real world case, it was a hand edit by the user to change the
+timezone field in an export. However, if it is good enough for hg, we
+have to make it good enough for git.
+
+Load commonly used test logic
+  $ . "$TESTDIR/testutil"
+
+  $ hg init hgrepo
+  $ cd hgrepo
+  $ touch beta
+  $ hg add beta
+  $ fn_hg_commit -m "test commit"
+  $ cat >patch2 <<EOF
+  > # HG changeset patch
+  > # User J. User <juser@example.com>
+  > # Date 1337962044 25201
+  > # Node ID 1111111111111111111111111111111111111111
+  > # Parent  0000000000000000000000000000000000000000
+  > Patch with sub-minute time zone
+  >
+  > diff --git a/alpha b/alpha
+  > new file mode 100644
+  > --- /dev/null
+  > +++ b/alpha
+  > @@ -0,0 +1,1 @@
+  > +alpha
+  > EOF
+  $ hg import patch2
+  applying patch2
+  $ hg gexport