Explicitly disable sparse checkout unless asked for (#1598)

When a worktree is reused by actions/checkout and the first time sparse checkout was enabled, we need to ensure that the second time it is only a sparse checkout if explicitly asked for. Otherwise, we need to disable the sparse checkout so that a full checkout is the outcome of this Action.

## Details
* If no `sparse-checkout` parameter is specified, disable it

This should allow users to reuse existing folders when running
`actions/checkout` where a previous run asked for a sparse checkout but
the current run does not ask for a sparse checkout.

This fixes https://github.com/actions/checkout/issues/1475

There are use cases in particular with non-ephemeral (self-hosted) runners where an
existing worktree (that has been initialized as a sparse checkout) is
reused in subsequent CI runs (where `actions/checkout` is run _without_
any `sparse-checkout` parameter).

In these scenarios, we need to make sure that the sparse checkout is
disabled before checking out the files.

### Also includes:

* npm run build
* ci: verify that an existing sparse checkout can be made unsparse
* Added a clarifying comment about test branches.
* `test-proxy` now uses newly-minted `test-ubuntu-git` container image from ghcr.io

---------

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Co-authored-by: John Wesley Walker III <81404201+jww3@users.noreply.github.com>

src/git-command-manager.ts

@@ -17,6 +17,7 @@ export interface IGitCommandManager {
   branchDelete(remote: boolean, branch: string): Promise<void>
   branchExists(remote: boolean, pattern: string): Promise<boolean>
   branchList(remote: boolean): Promise<string[]>
+  disableSparseCheckout(): Promise<void>
   sparseCheckout(sparseCheckout: string[]): Promise<void>
   sparseCheckoutNonConeMode(sparseCheckout: string[]): Promise<void>
   checkout(ref: string, startPoint: string): Promise<void>
@@ -171,6 +172,10 @@ class GitCommandManager {
     return result
   }
 
+  async disableSparseCheckout(): Promise<void> {
+    await this.execGit(['sparse-checkout', 'disable'])
+  }
+
   async sparseCheckout(sparseCheckout: string[]): Promise<void> {
     await this.execGit(['sparse-checkout', 'set', ...sparseCheckout])
   }

src/git-source-provider.ts

@@ -208,7 +208,9 @@ export async function getSource(settings: IGitSourceSettings): Promise<void> {
     }
 
     // Sparse checkout
-    if (settings.sparseCheckout) {
+    if (!settings.sparseCheckout) {
+      await git.disableSparseCheckout()
+    } else {
       core.startGroup('Setting up sparse checkout')
       if (settings.sparseCheckoutConeMode) {
         await git.sparseCheckout(settings.sparseCheckout)