Add new class SnoContext - resolves --repo consistently

[olsen232]

Lots of sno commands will need machine readable output (ie, json)...
... including reporting errors in a consistent, machine readable way.

In order that "non-existent repo" is reported in a consistent way, I have moved the non-existent repo error handling code into the context object itself. In a follow up change, I will make sure that this error is output using json when json flag is set.

At least two (very minor) bugs were fixed as a side effect of this:

• Some subcommands catch and re-raise a GitError as a click.BadParameter, but not all do (eg sno diff) - instead they test if the the repo returned is falsey, but this is not how pygit2 works.
• In cli.py, code tests for ctx.obj["repo_dir"] - but this string is always truthy, it defaults to "." - the result of this being always-true is that the flag "-C ." is always passed to the git executable, even if this is not the user's intent. (It doesn't seem to matter much though).

[rcoup]

Some subcommands catch and re-raise a GitError as a click.BadParameter, but not all do (eg sno diff) - instead they test if the the repo returned is falsey, but this is not how pygit2 works.

Yeah, some of this is leftovers from the initial GitPython prototype. Good catch 👍

[rcoup]

the flag "-C ." is always passed to the git executable, even if this is not the user's intent. (It doesn't seem to matter much though).

I think this is ok, means it won't pick up any unexpected GIT_DIR environment setting.

[rcoup]

This is great, cleans up a pile of inconsistent cruft