fix: avoid shallow-ifying full local clones

[yannrouillard]

Summary

Follow-up to #73. v0.58.0's git fetch --no-tags --depth=1 origin $TARGET_BRANCH runs unconditionally from the pre-push hook. On a full local clone (the typical dev setup), --depth=1 writes a .git/shallow file and truncates origin/$TARGET_BRANCH to a single commit, leaving the developer's local main detached from prior history. After a git push, users see weird states — rebases produce unexpected conflicts because the ancestor commits previously visible under origin/main have disappeared.

Reported by @tmaurin in the same Slack thread that triggered #73 (recovery: git fetch --unshallow). @dhalmschlager correctly diagnosed the mechanism and suggested the guard.

Reproduction (5 lines of shell on a full clone)

git clone <remote> /tmp/test && cd /tmp/test
test -f .git/shallow && echo "shallow BEFORE" || echo "not shallow BEFORE"
git fetch --no-tags --depth=1 origin main   # what v0.58.0 runs
test -f .git/shallow && echo "shallow AFTER" || echo "not shallow AFTER"
git log origin/main --oneline | wc -l       # now 1 instead of full history

Fix

Gate --depth=1 on whether the clone is already shallow:

if [ -f "$(git rev-parse --git-dir)/shallow" ]; then
    fetch_depth="--depth=1"
else
    fetch_depth=""
fi
git fetch --no-tags $fetch_depth origin "$TARGET_BRANCH"

CI (starts shallow via actions/checkout@v6 defaults) keeps the fast --depth=1 path; local full clones fetch normally and preserve their history.

Test plan

• New regression test in tests/test_git_branch_linearity.sh: runs the hook on a full local clone and asserts both that .git/shallow is absent and that origin/main retains its full 3-commit history.

• Confirmed the new test fails against v0.58.0 (origin/main has 1 commits, expected 3) and passes with this fix.

• Existing 4 tests from fix: make git-branch-linearity.sh work with shallow CI clones #73 still pass (6/6 total).

• Local validation on the real supply-demand-lng repo at the commit that originally failed (SHA 2778d4937c):

  Scenario	.git/shallow after hook	origin/main commits after hook
  Full clone + v0.58.0 (current)	❌ created	278 → 1 (bug reproduced)
  Full clone + this fix	✅ absent	278 → 278 (preserved)
  Shallow clone + this fix + GITHUB_HEAD_REF	✅ stays shallow	exit 0

• End-to-end CI validation in the real repo that was broken. Reopened the failing state of PR #122 (SHA 2778d4937c) with a single change — rev: v0.57.0 → this PR's fix commit — then ran the pre-commit workflow in both actions/checkout configurations:

  Configuration	actions/checkout	Result
  Shallow clone (default)	fetch-depth: 1	✅ Check for git branch linearity... Passed (job)
  Full clone	fetch-depth: 0	✅ Check for git branch linearity... Passed (job)

  Validation PRs (now closed): Kpler/supply-demand-lng#124 (shallow) and Kpler/supply-demand-lng#125 (full).