fix sourcemap of prepend when hire false

[yiminghe]

when prepend, the generated map is not recognized by chrome/firefox: can not set break point on second line when there are multiple lines

[mourner]

Thank you!

[petebacondarwin]

I think that this fix is not entirely correct.
Now the source map contains a mapping for every line of a chunk, even though nothing has changed.

[petebacondarwin]

Each unedited chunk should only have a single mapping to mark the beginning of that chunk.
After this PR the source-map is bloated with new mappings for every line even though there is no need from the point of view of the source-map.

As an example consider:

> x = new MagicString('a\nb\nc');
> x.generateMap().mappings

'AAAA;AACA;AACA`

Notice that we have a multiline string which has had "no" operations on it but the source-map contains redundant mappings, which state that [0,0]->[0,0], [1,0]->[1,0] and [2,0]->[2,0].
The first mapping ([0,0]->[0,0]) is the only one that should be there.

Extrapolate this to a file with hundreds or thousands of lines and you get the picture.

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I am not quite sure what the specific problem that @yiminghe had. I don't see why the browser would not be able to deal with the source-map only containing AAAA.