ci_runner: conditionally set extension.partialClone based on filter

If the filter is empty, unset the config.
If the filter is available, set the config to "origin" to indicate the
remote is a promisor remote.

https://github.com/git/git/blob/08bdfd453584e489d5a551aecbdcb77584e1b958/Documentation/config/extensions.adoc?plain=1#L37-L47

enterprise/server/cmd/ci_runner/main.go

@@ -1963,10 +1963,6 @@ func (ws *workspace) config(ctx context.Context) error {
 		{"user.email", "[email protected]"},
 		{"user.name", "BuildBuddy"},
 		{"advice.detachedHead", "false"},
-		// With the version of git that we have installed in the CI runner
-		// image, --filter=blob:none requires the partialClone extension to be
-		// enabled.
-		{"extensions.partialClone", "true"},
 		// Disable this check for `git fetch` performance improvements
 		{"fetch.showForcedUpdates", "false"},
 		// Disable automatic gc - it can interfere with running `rm -rf .git` in
@@ -1981,6 +1977,16 @@ func (ws *workspace) config(ctx context.Context) error {
 		// displays a blocking popup dialog)
 		cfg = append(cfg, []string{"credential.helper", ""})
 	}
+	if len(*gitFetchFilters) > 0 {
+		// With the version of git that we have installed in the CI runner
+		// image, --filter=blob:none requires the partialClone extension to be
+		// enabled.
+		cfg = append(cfg, []string{"extensions.partialClone", "origin"})
+	} else {
+		if _, err := git(ctx, io.Discard, "config", "unset", "extensions.partialClone"); err != nil {
+			return err
+		}
+	}
 
 	writeCommandSummary(ws.log, "Configuring repository...")
 	for _, kv := range cfg {