[JENKINS-70094] Add support for partial clone

[serianox]

Closes #3771
Adds support of partial clone by providing a filterspec to a repository configuration

• performed by setting remote..promisor and remote..partialCloneFilter in the clone configuration
• detect changes requested of the filterspec and perform --refetch as needed
• revert to full clone if unsupported by the git implementation
• automatically detect during checkout if the repository is a partial clone and provides credentials if needed

This PR is I think more complete than #1131.

Testing done

Added functional tests

• test with a filter spec (on jgit, will revert to a test with a full clone)
• test with credentials
• test with changed filterspec

Currently being tested in a production server with a GB-sized repository

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[Copilot]

Pull request overview

Adds partial clone filter support to the git-client plugin by extending the Clone and Fetch command APIs and implementing CLI git behavior, with JGit falling back to full clone/fetch while warning. It also adds functional tests covering filter usage, filter changes, and credential use during checkout in partial-clone scenarios.

Changes:

• Add filter(String filterSpec) to CloneCommand and FetchCommand APIs.
• Implement partial-clone filter handling in CliGitAPIImpl (including --filter and --refetch behavior, plus checkout-time credential use for promisor remotes).
• Add functional tests for clone/fetch filters and partial-clone checkout with credentials; add JGit warnings when filter is requested.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/main/java/org/jenkinsci/plugins/gitclient/CliGitAPIImpl.java	Implements CLI git partial-clone filter support (fetch/clone config + checkout credential handling).
src/main/java/org/jenkinsci/plugins/gitclient/JGitAPIImpl.java	Adds no-op filter() implementations that warn and fall back to full behavior under JGit.
src/main/java/org/jenkinsci/plugins/gitclient/FetchCommand.java	Extends fetch API with filter(String) (needs Javadoc correction).
src/main/java/org/jenkinsci/plugins/gitclient/CloneCommand.java	Extends clone API with filter(String).
src/test/java/org/jenkinsci/plugins/gitclient/GitClientFetchTest.java	Adds functional tests validating fetch filter behavior and filter changes.
src/test/java/org/jenkinsci/plugins/gitclient/GitClientCloneTest.java	Adds functional test validating clone filter behavior (and promisor artifacts).
src/test/java/org/jenkinsci/plugins/gitclient/CredentialsTest.java	Adds functional test for checkout of partial clone that requires credentials.

Suppressed comments (2)

src/main/java/org/jenkinsci/plugins/gitclient/CliGitAPIImpl.java:620

• When CLI git is older than 2.22, this code silently skips adding --filter but still writes promisor/partialclonefilter config, which contradicts the PR intent to "revert to full clone if unsupported" and can incorrectly mark a full repo as a partial clone. Gate both args and config updates on version support and log a warning when unsupported.

                    if (isAtLeastVersion(2, 22, 0, 0)) {
                        args.add("--filter=" + filterSpec);
                    }

src/main/java/org/jenkinsci/plugins/gitclient/CliGitAPIImpl.java:923

• CloneCommand#execute uses git config --add for remote..promisor and remote..partialclonefilter. --add can create multiple values on repeated runs and make later reads ambiguous. Use a normal git config assignment, and avoid setting partial-clone config when CLI git is too old to support filters.

                if (filterSpec != null) {
                    launchCommand("config", "--add", "remote." + origin + ".promisor", "true");
                    launchCommand("config", "--add", "remote." + origin + ".partialclonefilter", filterSpec);
                }

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