g.extension: allow to install extensions from a fork of the official repo

[ninsbl]

This PR adds an o-flag to specify that the url points to a fork of the official repo. In combination with the branch option, this allows easy testing of changes in addons by others...

See #625

[tmszi]

Great work, I tested it and it works fine.

[ninsbl]

Thanks @tmszi for reviewing.

It is not necessary to define the relationship between the url and the branch option. Branch option has default answer main.

Yes. That is right. However, as it is right now, the branch option will be ignored if no url is given...
The official addon repository currently does not really have branches anyway (in contrast to feature branches in forks).

In the long run and for GRASS 8 we should probably aim at throwing out SVN and use the github/gitlab API to fetch addons (esp. as we already started using it for branches)...

Something along this lines:

curl https://api.github.com/repos/ninsbl/grass-addons/contents/grass7/imagery/i.sentinel/i.sentinel.import

This results in the following response (with a download URL for every file (folders have null)):

[
  {
    "name": "Makefile",
    "path": "grass7/imagery/i.sentinel/i.sentinel.import/Makefile",
    "sha": "50dfe89f5ecbd917b2e2d9019cf42bc55a4b23a0",
    "size": 115,
    "url": "https://api.github.com/repos/ninsbl/grass-addons/contents/grass7/imagery/i.sentinel/i.sentinel.import/Makefile?ref=master",
    "html_url": "https://github.com/ninsbl/grass-addons/blob/master/grass7/imagery/i.sentinel/i.sentinel.import/Makefile",
    "git_url": "https://api.github.com/repos/ninsbl/grass-addons/git/blobs/50dfe89f5ecbd917b2e2d9019cf42bc55a4b23a0",
    "download_url": "https://raw.githubusercontent.com/ninsbl/grass-addons/master/grass7/imagery/i.sentinel/i.sentinel.import/Makefile",
    "type": "file",
    "_links": {
      "self": "https://api.github.com/repos/ninsbl/grass-addons/contents/grass7/imagery/i.sentinel/i.sentinel.import/Makefile?ref=master",
      "git": "https://api.github.com/repos/ninsbl/grass-addons/git/blobs/50dfe89f5ecbd917b2e2d9019cf42bc55a4b23a0",
      "html": "https://github.com/ninsbl/grass-addons/blob/master/grass7/imagery/i.sentinel/i.sentinel.import/Makefile"
    }
  },
  {
    "name": "i.sentinel.import.html",
    "path": "grass7/imagery/i.sentinel/i.sentinel.import/i.sentinel.import.html",
    "sha": "e0e3826fb48c721523643144608799fb575e4f6d",
    "size": 8244,
    "url": "https://api.github.com/repos/ninsbl/grass-addons/contents/grass7/imagery/i.sentinel/i.sentinel.import/i.sentinel.import.html?ref=master",
    "html_url": "https://github.com/ninsbl/grass-addons/blob/master/grass7/imagery/i.sentinel/i.sentinel.import/i.sentinel.import.html",
    "git_url": "https://api.github.com/repos/ninsbl/grass-addons/git/blobs/e0e3826fb48c721523643144608799fb575e4f6d",
    "download_url": "https://raw.githubusercontent.com/ninsbl/grass-addons/master/grass7/imagery/i.sentinel/i.sentinel.import/i.sentinel.import.html",
    "type": "file",
    "_links": {
      "self": "https://api.github.com/repos/ninsbl/grass-addons/contents/grass7/imagery/i.sentinel/i.sentinel.import/i.sentinel.import.html?ref=master",
      "git": "https://api.github.com/repos/ninsbl/grass-addons/git/blobs/e0e3826fb48c721523643144608799fb575e4f6d",
      "html": "https://github.com/ninsbl/grass-addons/blob/master/grass7/imagery/i.sentinel/i.sentinel.import/i.sentinel.import.html"
    }
  },

But that is a different PR and more work...

[tmszi]

Thanks @tmszi for reviewing.

You are welcome.

It is not necessary to define the relationship between the url and the branch option. Branch option has default answer main.

Yes. That is right. However, as it is right now, the branch option will be ignored if no url is given...
The official addon repository currently does not really have branches anyway (in contrast to feature branches in forks).

Yes, you're right.

In the long run and for GRASS 8 we should probably aim at throwing out SVN and use the github/gitlab API to fetch addons (esp. as we already started using it for branches)...

We can use it, but the problem is that you need to download each file separately, number of files == number of requests/responses. It would be better if GitHub API had one REST endpoint to download any repository directory as an archive (zip or another format) with one request/response. I haven't found that yet. My opinion is that it will therefore be better to continue to use svn checkout.