I feel the original -------/+++++++ pair is slightly confusing because
each half can be a separator by itself. I don't know what character other
than '-'/'+' is preferred, but let's pick '%' (for "mod") per @martinvonz
suggestion.
I'm adding this mostly because it's useful for testing. That's also
the reason it supports displaying conflicts. I didn't call it `cat`
like `hg cat` because I haven't found `hg cat` on multiple files
useful.
