BugFix: Handling multiple changes in a singular transaction

[KsanStone]

When utilizing codemirror shortcuts such as alt+upArrow to move entire lines or editing via multiple cursors at once, a singular codemirror transaction may contain multiple changes in its change set, causing multiple am.splice calls.

This causes 2 issues:

1. am.splice modifies the provided path variable, causing it to be corrupted on subsequent iterations
2. The indices of the text document may have been altered by prior changes. Therefore, any subsequent modifications should utilize fromB instead of fromA, as it, unlike fromA, accounts for these index changes. Otherwise a RangeError may be thrown.

[alexjg]

Great spot, thanks for the PR! I'll also try and fix splice so that it doesn't mutate the path.