github: Keep git history intact in Change Log fragment check

The next step, Lint git commits, needs access git's history so it can
lint the relevant commits, otherwise it just lints all commits.
See #2707 for more details.

.changelog/2707.trivial.md

@@ -0,0 +1 @@
+github: Keep git history intact in Change Log fragment check

.github/workflows/ci-lint.yml

@@ -47,7 +47,7 @@ jobs:
           # Fetch the pull request' base branch so towncrier will be able to
           # compare the current branch with the base branch.
           # Source: https://github.com/actions/checkout/#fetch-all-branches.
-          git fetch --no-tags --prune --depth=1 origin +refs/heads/${BASE_BRANCH}:refs/remotes/origin/${BASE_BRANCH}
+          git fetch --no-tags origin +refs/heads/${BASE_BRANCH}:refs/remotes/origin/${BASE_BRANCH}
           towncrier check --compare-with origin/${BASE_BRANCH}
         env:
           BASE_BRANCH: ${{ github.base_ref }}