Make fetching source tarball by git commit more robust

[Flamefire]

(created using eb --new-pr)

I had a crate that was supposed to fetch a git commit from https://github.com/astral-sh/pubgrub. However the specified git commit 21ceec9a5fd2e8d6f71c3ea2999078fecbd13cbe was not available for git checkout after the clone.

My understanding is, that this commit is not reachable from the default (main) branch that is fetched, possibly after same rebase.

The approach done here is to add a separate git fetch call to fetch the specified commit. That resolves the issue in my case.

We could go even further and don't use git clone at all:

mkdir repo
cd repo
git init
git fetch --depth=1 https://github.com/... sha/branch/tag [--recursive]
git checkout sha/branch/tag