Thoughts on support for vc-diff unsaved buffer contents?

[Campbell Barton]

Hi, recently I've run into a situation where it's necessary to calculate 
modified line ranges without first saving the buffer (where "modified" 
is determined by a version control diff).

I'm curious if this is something that might be supported - or, is 
already supported (although I'm fairly sure it's not).

The use case is for auto-formatters only to format modified lines.

This particular case is for clang-format.el although the same 
functionality could useful for other formatters too.

clang-format.el can be configured to format on save,
so when saving: clang-format.el calculates the lines that differ from 
the working copy them formats them before saving.

Currently it's necessary for clang-format.el to call git & diff 
directly, which makes the functionality limited to git. I was hoping 
this could be supported in a generic way that uses generic `vc` API's.