Add support of git protocol over https for fetch

[pbabinca]

Currently for URLs specified in <fetch /> restraint either:

• fetches archive with libcurl when scheme is https, http or file
• uses git protocol to request specific reference (branch/tag) and file path and only those are fetched as an archive, when scheme is git.

For large repositories selective fetch has much better performance than download of the whole contents. The git protocol does not use encryption. To address requirements of the of data over-the-wire and not to make the fetching less performant I would like to request new feature - fetch URL over https using git protocol.

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My understanding is that these code blocks determines the protocol to use fetch the archive:

• src/task.c#L148-L157
• src/dependency.c#L495-L502

and I'd suggest to use a new scheme: git+https for these URLs.

fetch_git.c would need to be extended to support, hopefully, small subset of https://github.com/git/git/blob/master/Documentation/gitprotocol-http.txt .

[StykMartin]

Support for git+https certainly sounds reasonable, we just need to make sure we can unpack properly, as this has been problematic for many attempts in the past.