OpenHandler and ResourceProvider for git

[svenefftinge]

As part of #165, we need to register an ResourceProvider

That can be used to open editors and provide resource contents for files from specific git revisions.
For the URI I propose the following:

gitrev://repo_name/my/file.tsx#HEAD
\_____/\__________/\__________/\______/
   |         |          |           | 
scheme   repository    path      commitish

[hexa00]

I think handling #HEAD might be tricky since a file can end with #...

Rather then mitigate this maybe we could have:

gitrev://repo_name/commithash/my/file.ts 

Then we always use the same separator...

Github does somthing similar like:

https://github.com/theia/blob/db5745a1089fdbfb73c12ac90165bd70dfe1b2a7/packages/terminal/src/browser/terminal-widget.ts

to view a file

[svenefftinge]

Files ending with # will cause problems with URIs in general. It is not limited to this style of uri.
But if we require a gitrev to come with a fragment, it would even work with hashes in file names, as only the last hash is considered (AFAIR)

[svenefftinge]

But thinking about it, as the whole repo is available for a certain revision, we could indeed make it the first segment after the repo.

[akosyakov]

In URI query params are always starts from ?: https://en.wikipedia.org/wiki/Query_string

[akosyakov]

In the file path ? should be encoded. Our URI takes care about it.

[svenefftinge]

Fixed with #392