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.

adding foo
adding bomb
adding a.c
adding dir/a.o
adding dir/b.o
! a.c
! dir/a.o
! dir/b.o
? .hgignore
a.c: unsupported file type (type is fifo)
! a.c