plan index.version for when we can write V4 indices.

Note that it's likely that we need something like `desired_version`
as option before V4 indices are supported even when we start writing
indices for commits, for example. By then we also want to support
all extensions, so a lot of work needed for proper support.

src/plumbing/progress.rs

@@ -526,7 +526,7 @@ static GIT_CONFIG: &[Record] = &[
     },
     Record {
         config: "core.splitIndex",
-        usage: Planned { note: Some("needed to be able to read split indices (mandatory extension)") },
+        usage: NotPlanned { reason: "we don't want to be able to create split indices, but we will read them. It's (somewhat) superseded by sparse indices" },
     },
     Record {
         config: "splitIndex.maxPercentageChange",
@@ -540,6 +540,10 @@ static GIT_CONFIG: &[Record] = &[
         config: "index.sparse",
         usage: Planned { note: Some("together with 'core.sparseCheckout' and 'core.sparseCheckoutCone', configures if the index should be written sparse or not") },
     },
+    Record {
+        config: "index.version",
+        usage: Planned { note: Some("once V4 indices can be written, we need to be able to set a desired version. For now we write the smallest possible index version only.") },
+    },
     Record {
         config: "sparse.expectFilesOutsideOfPatterns",
         usage: NotPlanned { reason: "todo" },