Previously tabs were configured to all use the same width. However, the
tab width setting is supposed to configure the distance between
tab-stops, not the width of the tabs themselves. Additionally, the width
of a tab element as it's inserted into diff content should be the width
needed to get to the next tab-stop.
Reconfigures the `_textLength` and `_addTabWrappers` methods to respect
this aspect of tab behavior.
Notably, `_addTabWrappers` formerly accepted an HTML string as input,
and was called after intraline differences were applied. With this
change it acts only on raw (non-HTML) strings so that it can directly
determine the position of the tab within the line, and it is called
before intraline differences are applied.
Bug: Issue 4252
Change-Id: I44826d917a505a245fd2b20ccf0ac19378f2806c
812f225b79