Merge Editor: fix a bug in LineRange.join(other)

[pisv]

It looks like there is a significant bug lurking in the LineRange.join(other) method.

When other.startLineNumber < this.startLineNumber, the line count of the resulting line range should be

Math.max(this.endLineNumberExclusive, other.endLineNumberExclusive) - other.startLineNumber

rather than

Math.max(this.endLineNumberExclusive, other.endLineNumberExclusive) - this.startLineNumber

[hediet]

Thanks!

This LineRange class has to go eventuall in favor of the core LineRange class (which uses [startLineNumber, endLineNumberExclusive) - using the length was a mistake, but I thought I could use it to avoid the exclusive/inclusive problem.)