The parameters displayed in "Unknown fast forward strategy" and "Unknown merge analysis" were swapped. This change fixes that.

These were deprecated for the release of v0.23 so let's get rid of them for the
next release.

This is in preparation for a portable project alongside the net40 one.

This deprecates them from the repository itself and makes it clear this
isn't a fundamental operation but something which builds on them.

These commands work on a repository, and we can leave the Index be the
data structure. Thus it is no longer written to disk after every change.

This exposes what we've only had internally, letting you ask for e.g. a
tree from a tag.

Make use of this internally as well instead of the special-cased method
we had up to now.

We add an implicit conversion from the handle to the pointer as there
are a lot of places which rely on the equivalent functionality for the
SafeHandle.

Instead of throwing an exception, return the merge error in the normal
way, as this early failure was requested by the user and thus not an
exceptional circunstance.

The public methods use the disposable wrapper so we don't have to mark
them as unsafe.

A hard reset can take a while as it involves a checkout, allow use of progress reporting.

I wasn't sure what happens if no repo is found. (does it return null or throws  an exception).
So I thought it could be stated clearer for people like me.

returned by remote ls

Fix #818

Cleaning up white space usage

Cleaning up `using` statements

Cleaning up `cref` usage

Cleaning up parameter alignment

Cleaning up brace positioning

Introduce conflict data to status and diff information.  Introduce
staging of conflicts.

https://github.com/libgit2/libgit2/compare/9bbc8f3...ff8d635

Fix #527