CI: bump-deployments - refresh index before checking for changes

[neddp]

Problem

The bump-deployments task fails with exit 1 even when the submodules are already pinned to the desired commits:

+ git diff-index --quiet HEAD
+ git commit -am 'Update deployments'
On branch bump-deployments-ci
nothing to commit, working tree clean

Root cause

The task rewrites deployment-versions.txt with echo > on every run. After Concourse's shallow clone (which resets file mtimes), the file content is byte-identical but its stat metadata differs from git's cached stat. git diff-index --quiet HEAD trusts the stale stat cache and reports the file as modified (blob SHA 0000000…), so the || git commit branch runs — but git commit refreshes the index internally, finds no real change, and exits 1, failing the task.
Isolate in: https://bosh.ci.cloudfoundry.org/builds/373771172

Fix

Run git update-index -q --refresh before the check. This re-hashes stat-dirty files and, when the content is unchanged, clears the dirty flag — so git diff-index --quiet gets an accurate answer and the commit is correctly skipped. A genuine submodule bump still commits normally.

Verified in: https://bosh.ci.cloudfoundry.org/builds/373773086

[coderabbitai]

Walkthrough

The deployment bump task now refreshes the Git index quietly before git diff-index checks for changes.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: refreshing the Git index before checking for deployment changes.
Description check	✅ Passed	The description explains the failure, root cause, fix, and verification, all of which directly relate to the changeset.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-bump-deployments-stat-cache

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[ramonskie]

related to #691