hgmerge: add cleanup functions This patch adds functions cleanup, success and failure. The last two of these should be used instead of exit. Current code was changed to use them. It also moves $HGTMP to the top of the file (it's used in the cleanup function), changes the comment and removes now unneeded trap in the diff+patch merge.

port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export