Git fetcher: Don't create refs when fetching by revision (backport #12386)

[mergify]

Motivation

This fixes a couple of issues with the Git fetcher:

• When fetching by revision, don't create a ref named refs/head/<revision>. (This was caused by doing git fetch <revision>:<revision>.) This is unnecessary and triggers the next bug.

• When parsing the output of git ls-remote --symref, make sure that the first line references HEAD, otherwise a line like

  5c4410e3b9891c05ab40d723de78c6f0be45ad30        refs/heads/5c4410e3b9891c05ab40d723de78c6f0be45ad30
  

  will cause 5c44... (created by the previous bug) to be interpreted as a ref, leading to errors like

  fatal: Refusing to point HEAD outside of refs/
  warning: could not update cached head 'd275d93aa0bb8a004939b2f1e87f559f989453be' for 'file:///tmp/repo'
  

• Don't pass URL query parameters for file:// URLs. Git interprets them as part of the file name, so passing parameters like rev breaks. Only relevant for testing (when _NIX_FORCE_HTTP is set) and local bare repos.

Context

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This is an automatic backport of pull request #12386 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 9f72d5b has failed:

On branch mergify/bp/2.25-maintenance/pr-12386
Your branch is up to date with 'origin/2.25-maintenance'.

You are currently cherry-picking commit 9f72d5bce.
  (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")

Cherry-pick of ee9fa0d has failed:

On branch mergify/bp/2.25-maintenance/pr-12386
Your branch is ahead of 'origin/2.25-maintenance' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit ee9fa0d36.
  (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

[edolstra]

Closing as not applicable.