Move Repository.Reset(paths) into Index

[dahlbyk]

As proposed in #799:

• Move Repository.Reset(…, paths, …) and related extensions to Index
• Add Repository.ResetPaths(…) and related extensions, deferring to new Index methods
• Mark Repository.Reset(…, paths, …) and related extensions as [Obsolete], referring to either ResetPaths or Index.Reset.

[nulltoken]

@alexandrudima Now that #799 is merged, would you be interested in making this one happen?

[bording]

Looks like this still needs to be done. I'll take a look at it.

[nulltoken]

🆒 Labeled as in-progress then