fix(): error context for git_fetch refspec not found

[dacianpascu06]

What does this PR try to resolve?

Fixes Issue : #14621

Adds more error context for fetching a commit that doesn't exists.

How should we test and review this PR?

I've created two tests, one for fast_path and one for libgit2.

r? @weihanglo

[rustbot]

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[weihanglo]

It is interesting that the error message doesn't seem to be emitted on GitHub Actions machine, but only on my local machine.

If it still happens, instead of asserting the entire message, try omitting some parts we're not interested in with ... (For the syntax, see https://doc.rust-lang.org/nightly/nightly-rustc/cargo_test_support/compare/index.html).

Suggested change

	[UPDATING] git repository `https://github.com/rust-lang/bitflags.git`
	[WARNING] spurious network error (3 tries remaining): could not read from remote repository; class=Net (12); code=Eof (-20)
	[WARNING] spurious network error (2 tries remaining): could not read from remote repository; class=Net (12); code=Eof (-20)
	[WARNING] spurious network error (1 tries remaining): could not read from remote repository; class=Net (12); code=Eof (-20)
	[ERROR] failed to get `bitflags` as a dependency of package `foo v0.1.0 ([ROOT]/foo)`
	Caused by:
	failed to load source for dependency `bitflags`
	Caused by:
	Unable to update https://github.com/rust-lang/bitflags.git?rev=11111b376b93484341c68fbca3ca110ae5cd2790
	Caused by:
	failed to clone into: [ROOT]/home/.cargo/git/db/bitflags-[HASH]
	Caused by:
	revision 11111b376b93484341c68fbca3ca110ae5cd2790 not found
	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:
	could not read from remote repository; class=Net (12); code=Eof (-20)
	[UPDATING] git repository `https://github.com/rust-lang/bitflags.git`
	...
	[ERROR] failed to get `bitflags` as a dependency of package `foo v0.1.0 ([ROOT]/foo)`
	Caused by:
	failed to load source for dependency `bitflags`
	Caused by:
	Unable to update https://github.com/rust-lang/bitflags.git?rev=11111b376b93484341c68fbca3ca110ae5cd2790
	Caused by:
	failed to clone into: [ROOT]/home/.cargo/git/db/bitflags-[HASH]
	Caused by:
	revision 11111b376b93484341c68fbca3ca110ae5cd2790 not found
	...

[weihanglo]

For the context:

---- git::git_fetch_libgit2_error_message stdout ----
running `/Users/runner/work/cargo/cargo/target/debug/cargo fetch`
thread 'git::git_fetch_libgit2_error_message' panicked at tests/testsuite/git.rs:4163:10:

---- expected: tests/testsuite/git.rs:4135:27
++++ actual:   stderr
   1    1 | [UPDATING] git repository `[https://github.com/rust-lang/bitflags.git`](https://github.com/rust-lang/bitflags.git%60)
   2      - [WARNING] spurious network error (3 tries remaining): could not read from remote repository; class=Net (12); code=Eof (-20)
   3      - [WARNING] spurious network error (2 tries remaining): could not read from remote repository; class=Net (12); code=Eof (-20)
   4      - [WARNING] spurious network error (1 tries remaining): could not read from remote repository; class=Net (12); code=Eof (-20)
   5    2 | [ERROR] failed to get `bitflags` as a dependency of package `foo v0.1.0 ([ROOT]/foo)`
   6    3 | 
   7    4 | Caused by:
   8    5 |   failed to load source for dependency `bitflags`
   9    6 | 
          ⋮
  15   12 | 
  16   13 | Caused by:
  17   14 |   revision 11111b376b93484341c68fbca3ca110ae5cd2790 not found
  18   15 | 
  19   16 | Caused by:
  20      -   network failure seems to have happened
  21      -   if a proxy or similar is necessary `net.git-fetch-with-cli` may help here
  22      -   https://doc.rust-lang.org/cargo/reference/config.html#netgit-fetch-with-cli
       17 +   Could not decode server reply
  23   18 | 
  24   19 | Caused by:
  25      -   could not read from remote repository; class=Net (12); code=Eof (-20)
       20 +   Unknown or unsupported header: "ERR upload-pack: not our ref 11111b376b93484341c68fbca3ca110ae5cd2790"
       ```

[weihanglo]

Thanks!