git-svn: ignore changeless commits when checking for a cherry-pick

git-svn: ignore changeless commits when checking for a cherry-pick

Update git-svn to ignore commits that do not change the tree when it is
deciding if an svn merge ticket represents a real branch merge or just a
cherry-pick.

Consider the following integration model in the svn repository:

   F---G  branch1
  /     \
 D  tag1 \   E  tag2
/         \ /
A---B      C  trunk

branch1 is merged to trunk in commit C.

With this patch, git-svn will correctly identify branch1 as a proper merge
parent, instead of incorrectly ignoring it as a cherry-pick.

Signed-off-by: Andrew Myrick <amyrick@apple.com>
Acked-by: Sam Vilain <sam@vilain.net>
Acked-by: Eric Wong <normalperson@yhbt.net>