A diff was found where a line with 3 tabs was unindented 1 tab, had

1 tab removed, and had the last tab replaced with spaces. This caused

the unindentation to show 3 tabs removed, which was wrong.
To fix this, we have to have a little more state. We needed to know not

only the raw number of indentation characters that changed, but also the

normalized (tabs to spaces) difference in indentation between the lines.
With this information, the serializers could figure out when we've

actually hit the number of indentation normalized to spaces, in order to

stop processing the string. They then needed to be modified to return

the remaining portion along with the serialized version.
After this change, a more complex indentation change like

\t\t\t to \t<8 spaces> will show the right thing.

Tested with a repro case based on a diff found in the wild. Before this

change, it showed the wrong number of unindented tabs. After, it showed

the correct number.
Added a couple unit tests, which passed.
All other unit tests passed as well. I had to modify the parameters going into

some functions, but I set them based on what I expected the values to be, rather

than what the unit tests failed with, and they matched what the code actually

computed.