Fixes to "hg heads -r FOO": Make it actually work (undefined variable 'rev'; allow to pass a rev parameter). repo.branchlookup() doesn't need a copy of heads because it doesn't modify it. Use None as default argument to heads() instead of nullid. Doc string PEPification.

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