fix: check if rev is full commit sha for github fast path

[weihanglo]

What does this PR try to resolve?

This adds a check when parsing revision as an oid.
libgit2 zero-pads revisions, which we've seen in #13188,
and was introduced by #13946

How should we test and review this PR?

Testings GitHub specific code path needs more efforts, so I didn't include any.
Here are manual test steps.

cargo new foo --lib
cd foo
cargo add --git https://github.com/weihanglo/empty-library --rev 21d408eccb
rm -rf "~/.cargo/git/{checkouts,db}/empty-library-*"
cargo +nightly fetch # expect to fail
./target/debug/cargo fetch # expect to work
CARGO_NET_GIT_FETCH_WITH_CLI=true ./target/debug/cargo fetch --verbose # expect work

Additional information

Fixes #13968

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[epage]

@bors r+

[bors]

📌 Commit 26c88db has been approved by epage

It is now in the queue for this repository.

[bors]

⌛ Testing commit 26c88db with merge 95eeafa...

[bors]

☀️ Test successful - checks-actions
Approved by: epage
Pushing 95eeafa to master...