fetchers/git: make relative path absolute for local repo (backport #12107)

[mergify]

Motivation

Closes #9708 (again).

This patch:

• (naively) Resolves relative paths to local git repos, returning absolute paths, thus allowing flake URIs such as git+file:./${submodule} to continue working.
• Adds tests to prevent future breakages.

Context

Flake URIs such as git+file:./${submodule} used to work before 2.18, but broke in 2.19, was fixed later, and broke again recently since 3e0129c, resulting in a core-dumped assertion error in #9708 (comment).

This PR suppresses the error and allows git+file:./${submodule}to continue working. However, this fix is not ideal and should potentially be superseded by a better submodule implementation in the future. See:

• Flake input git+file:./${submodule} no longer works #9708
• libutil/url: fix git+file:./ parse error #9897

A better fix is outlined by @roberth in https://discourse.nixos.org/t/57783, namely:

This bug seems to have allowed relative git flake inputs to be resolved against the current working directory (as in POSIX), and this tends to work out ok in the context of flakes, but is the wrong behavior, as it should resolve against the flake.nix base directory instead.
Supporting relative paths in fetchTree would add significant value.
This behavior was used as a workaround for lacking relative flake input or “subflake” support.

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This is an automatic backport of pull request #12107 done by [Mergify](https://mergify.com).