patch: reapply path limit

This was originally applied added in commit eac1b67 (ui-patch: Apply
path limit to generated patch, 2010-06-10) but the ability to limit
patches to particular paths was lost in commit 455b598 (ui-patch.c: Use
log_tree_commit() to generate diffs, 2013-08-20).

The new output is slightly different from the original because Git's
diff infrastructure doesn't give us a way to insert an annotation
immediately after the "---" separator, so the commit has moved below the
diff stat.

Signed-off-by: John Keeping <john@keeping.me.uk>