Fix wrong indenting with hard tabs in binop pairs

[matthewhughes934]

Note: opening as draft for a discussion. I'm not sure about the approach since there might be wide implications for all other callers of last_line_width

---

When rewriting pairs of multi-line binary operations, we would check the length of a line to see if we can snuggle the current line into the previous one. We compute the length of the previous line using last_line_width which is not aware of indentation widths (i.e. config.tab_spaces), so if we were formatting something like:

if some_long_name {
        foo
    } | if some_other_name {
        bar
    }

Then when we get to the line for | if some_other_name { we check the previous line, which is:

• `\s\s\s\s\s\s\s\s}' if we are not using hard tabs (and 1 tab = 4 spaces), and
• \t\t} if we are using hard-tabs

last_line_width would return 9 for the first one, and 3 for the second. Meaning if we're using hard tabs we could conclude that it should fit on the previous line, leading to inconsistent behaviour between the two.

To fix this, create a version of last_line_width that is aware of config.tab_spaces.