fix: add error message for github PR url in dep

[joshka]

Prior to this, using a github PR URL would cause cargo to attempt to
fetch from an incorrect URL several times before failing.
Providing a github pull request url now fails with an error message
that shows how to fix the problem.

E.g.:

bar = { git = "https://github.com/foo/bar/pull/123" }

Now gives the following error message:

dependency (bar) specifies a GitHub pull request link. If you were trying to specify a specific github PR, replace the URL with the git URL (e.g. `git = "https://github.com/foo/bar.git"`) and add `rev = "refs/pull/123/head"` in the dependency declaration.

Fixes: #15001

[rustbot]

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[weihanglo]

Thanks! If needed, we can iterate it later :)

[joshka]

I'm not quite sure what the right approach to getting the tests to succeed both in CI and locally for this change. Any guidance?

[weihanglo]

Interesting. I thought it was network flakiness. Anyway, you can use ... to omit the unimportant part.

Suggested change

	[WARNING] spurious network error (3 tries remaining): unexpected http status code: 404; class=Http (34)
	[WARNING] spurious network error (2 tries remaining): unexpected http status code: 404; class=Http (34)
	[WARNING] spurious network error (1 tries remaining): unexpected http status code: 404; class=Http (34)
	[ERROR] failed to get `bar` as a dependency of package `foo v0.0.0 ([ROOT]/foo)`
	Caused by:
	failed to load source for dependency `bar`
	Caused by:
	Unable to update https://github.com/foo/bar/pull/123
	Caused by:
	failed to clone into: [ROOT]/home/.cargo/git/db/123-[HASH]
	Caused by:
	network failure seems to have happened
	if a proxy or similar is necessary `net.git-fetch-with-cli` may help here
	https://doc.rust-lang.org/cargo/reference/config.html#netgit-fetch-with-cli
	Caused by:
	unexpected http status code: 404; class=Http (34)
	...
	[ERROR] failed to get `bar` as a dependency of package `foo v0.0.0 ([ROOT]/foo)`
	Caused by:
	failed to load source for dependency `bar`
	Caused by:
	Unable to update https://github.com/foo/bar/pull/123
	...

[joshka]

This is the problem with not making this change cause an error. If this is a warning, then after emitting the warning, cargo will attempt to load the github PR url, which is not a git repo, and so that attempt will 404 or some other error (e.g. not a git repo). I think that ... change would make sense though, so committing it as is.

PS. feel free to squash this commit (I know Ed prefers atomic commits in the shape that will end up in the repo, but this is really a single change - not sure how that applies to this project).

[weihanglo]

[patch] test also needs the same trick :)

[epage]

Sorry I've not been able to get back to this and thanks @weihanglo for picking it up.

A non-blocking thought (since some improvement is better than no improvement) is to offer something similar for cargo add --git URL