histedit: Speed up scrolling in patch view mode

Store patchcontents into the mode state, avoiding the expensive
call to ui for computing the patchcontents.

Before this change in large repos histedit patch view mode can
be very irresponsive.

hgext/histedit.py

@@ -1079,6 +1079,8 b' def movecursor(state, oldpos, newpos):'
 def changemode(state, mode):
     curmode, _ = state['mode']
     state['mode'] = (mode, curmode)
+    if mode == MODE_PATCH:
+        state['modes'][MODE_PATCH]['patchcontents'] = patchcontents(state)
 
 def makeselection(state, pos):
     state['selected'] = pos
@@ -1134,7 +1136,7 b' def changeview(state, delta, unit):'
     if mode != MODE_PATCH:
         return
     mode_state = state['modes'][mode]
-    num_lines = len(patchcontents(state))
+    num_lines = len(mode_state['patchcontents'])
     page_height = state['page_height']
     unit = page_height if unit == 'page' else 1
     num_pages = 1 + (num_lines - 1) / page_height
@@ -1394,7 +1396,8 b' pgup/K: move patch up, pgdn/J: move patc'
 
     def renderpatch(win, state):
         start = state['modes'][MODE_PATCH]['line_offset']
-        renderstring(win, state, patchcontents(state)[start:], diffcolors=True)
+        content = state['modes'][MODE_PATCH]['patchcontents']
+        renderstring(win, state, content[start:], diffcolors=True)
 
     def layout(mode):
         maxy, maxx = stdscr.getmaxyx()