avoid converting custom indexes to pandas indexes when formatting coordinate diffs

[keewis]

This would attempt to convert custom indexes to pandas indexes (by calling .indexes instead of .xindexes), which for some implementations may raise.

(I need this for xarray-contrib/pint-xarray#163)

• [x] extracted from #8111
• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

[TomNicholas]

Will this fix #5812? Or not quite yet?

[keewis]

not yet, no. This just makes sure that the formatting of the coordinates representation does not call .indexes.