Removed now obsolete min/max check in walkchangerevs().

Replaced fixed window size for walkchangerevs with an increasing one. Window sizes starts at 8 (for good interactiveness) and doubles with each window until it is 512, which seems to be the maximum efficient value.

hgmerge: do not use file with markers in tools merging themselves This time I forgot about a crucial part of the logic change. The patch makes kdiff3 and tkdiff useful again.

Fix hgmerge: mkdir "'/tmp'/hgmerge..." and ask_if_merged() didn't work.