generate forge URL for sharing from line-number-at-pos

[gvoysey]

Very often, I want to share a link to a forge's website (company gitlab, github, etc.) to a section of code that I have open in emacs. Right now, I:

• have repo.git open in emacs with a branch checked out.
• have repo.git/src/foo.py open in emacs, and know i want to point someone at lines 3 through 8
• go to https://github.com/owner/repo.git and click around until i get a link like https://github.com/owner/repo.git/blob/branch/src/foo.py#L3-8
• share this link
• laboriously reacquire context in emacs.

What i'd like:

• have repo.git open in emacs with a branch checked out.
• have repo.git/src/foo.py open in emacs, and know i want to point someone at lines 3 through 8
• run something like magit-forge-link-dwim, perhaps with some prefix args or a transient to help
• copy the link that just got put in my killring and share it
• just keep working and leave the browser out of it.

Subtleties where a transient would be nice:

• select between multiple remotes in the case of multiple forges
• fully resolved URL (with embedded commit hash) if possible, or otherwise use the branch url

Just because this involves constructing a URL whose format is different between {gitlab, github, gitea, ...}, I figure this goes into forge, not base magit -- but i could be wrong.

[gvoysey]

#91 whoops, dupe.