Ignore empty lines when parsing git-rebase-todo

Ignore empty lines when parsing git-rebase-todo

When starting a rebase with C Git, there may be empty lines in the
git-rebase-todo file. Before this change, JGit would fail to parse the
file with e.g. the following exception:

  JGitInternalException: Unknown or unsupported command "
  #", only "pick" is allowed.

This happened when there was an empty line just before the comments,
because the nextSpace would be the one from the comment. Now the empty
lines are ignored by checking for nextSpace < ptr outside of the loop.

Change-Id: I94ad299f367c846e7729c74f49c6b8f93f75ae81
Signed-off-by: Robin Stocker <robin@nibor.org>