Improve errors when we are trying to access a git repository with partial history (+ fix fetchGit on these repos) (backport #13265)

[mergify]

Motivation

This is a subset of #13260 but doesn't change the default behavior for flake urls, so it can be backported.

Error fixed by this PR:

error:
       … while calling the 'fetchGit' builtin
         at «string»:1:2:
            1| (builtins.fetchGit { url = "file:///tmp/nix-test/main/fetchGitShallow/shallow-worktree"; shallow = true; }).rev
             |  ^

       … while fetching the input 'git+file:///tmp/nix-test/main/fetchGitShallow/shallow-worktree?exportIgnore=1&shallow=1'

       error: getting parent of Git commit 'b84683a868d74413ba54bd3c7da8bd57a94a8c8a': object not found - no match for id (cfad7cc96b05c722c83ea7c41269a6910c0a2020)

Context

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This is an automatic backport of pull request #13265 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 0479db9 has failed:

On branch mergify/bp/2.28-maintenance/pr-13265
Your branch is up to date with 'origin/2.28-maintenance'.

You are currently cherry-picking commit 0479db934.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   src/libfetchers/git.cc

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally