run merge program in repo root.
before, merge program was run in user current directory. Well defined
location is better for scripts. no more need to pass HG_ROOT to merge
program now.
Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory