histedit: fix diff colors

The problem here is that indexing a bytestring gives you integers, not
chars, so the comparison to b'+' ends up being wrong.

We don't really have a way to test curses output, so no tests to
verify the correctness of this behaviour.

hgext/histedit.py

@@ -1427,11 +1427,11 b' pgup/K: move patch up, pgdn/J: move patc'
         for y in range(0, length):
             line = output[y]
             if diffcolors:
-                if line and line[0] == b'+':
+                if line.startswith(b'+'):
                     win.addstr(
                         y, 0, line, curses.color_pair(COLOR_DIFF_ADD_LINE)
                     )
-                elif line and line[0] == b'-':
+                elif line.startswith(b'-'):
                     win.addstr(
                         y, 0, line, curses.color_pair(COLOR_DIFF_DEL_LINE)
                     )