Bugfix for diff hunks beginning with a + line.

These do not normally occur in git diff output, but other-hunk
generation when splitting can produce them.  For example, if a hunk
looks like

	@@ -3866,6 +3863,27 @@ static void show_playlist(void)
	   }
	 }

	+static void show_info(CD *cd, TG *tg)
	+{
	+ void tg_chain(TG *tg)
	+  { if (!tg || !tg->parent) return;
	+    tg_chain(tg->parent);
	+    scroll_space("%s",tg->text);
	+  }
	+
	+ scroll_space(cd->text);
	+ tg_chain(tg);
	+}
	+
	+static void reset_pending_playlist(void)
	+{
	+ int i;
	+
	+ send_player_head(PLAYER_M2P_FLUSH_PENDING);
	+ for (i=1;i<playlist.n;i++) tell_player_play(playlist.v[i]->cd,playlist.v[i]->tg);
	+ ignore_done_to_flush = 1;
	+}
	+
	 static void dl_cds_init(void)
	 {
	  cdvlist.dn = 0;

and the user splits off the addition of show_info() as the first part,
the generated second part looks like

	@@ -3866,15 +3863,27 (split @@ -3866,6 +3863,27)
	+   }
	+ }
	+
	+ static void show_info(CD *cd, TG *tg)
	+ {
	+  void tg_chain(TG *tg)
	+   { if (!tg || !tg->parent) return;
	+     tg_chain(tg->parent);
	+     scroll_space("%s",tg->text);
	    }
	+
	+  scroll_space(cd->text);
	+  tg_chain(tg);
	  }

	  static void reset_pending_playlist(void)
	  {
	   int i;

which tickles this bug.  We might be able to get away with just
arranging for diff generation to never produce such hunks, but I prefer
to make the application code robust in the face of them rather than
just hoping they can never occur.