Bundle-uri capability: Only advertise when bundle exist

This MR is a building block towards enabling bundle-uri feature.

Here we modify the way bundle-uri capability is advertised to the client to only make it so when a bundle exist for the given repository.

For SSH, this change does not provide much benefit since the whole packfile negociation flow is performed on a single connection. In other words, the flow starts when the client starts the upload-pack service, and it ends when the client received all the objects it needed.

For SmartHTTP, however, this is different. Each request/command sent by the client (ls-refs, bundle-uri, fetch) is sent on different, stateless, RPC calls. There is no way to know what command the client is sending to the git-upload-pack process until that process is actually started, which means that the Git config injected into the process must be computed in advance for every request; this implies computing the SignedURL of the bundle if it exist. By advertising bundle-uri capability only when a bundle exists for the given repository, we make sure the client won't send a bundle-uri command if no bundle exist.

This not only reduces by 1 the number of round-trip request and the number of Git config computation, but it also make it easier to monitor the use of bundle-uri feature, since now we can be sure that when a client sends the bundle-uri command, it is because:

1. A bundle exists
2. The server advertised the capability
3. The client support the capability

Thus, we now have a way to know if a client is using bundle-uri or not.

References: #6572 (closed)

internal/bundleuri/git_config.go

@@ -3,9 +3,9 @@ package bundleuri
 import (
 	"context"
 	"fmt"
-	"gitlab.com/gitlab-org/gitaly/v16/internal/featureflag"
 	"strconv"
 
+	"gitlab.com/gitlab-org/gitaly/v16/internal/featureflag"
 	"gitlab.com/gitlab-org/gitaly/v16/internal/git/gitcmd"
 )
 
@@ -21,18 +21,15 @@ import (
 // This function is also used when spawning git-upload-pack(1) --advertise-refs in
 // response to the GET /info/refs request.
 func CapabilitiesGitConfig(ctx context.Context, enable bool) []gitcmd.ConfigPair {
-	cfg := func(enable bool) []gitcmd.ConfigPair {
-		return []gitcmd.ConfigPair{
-			{
-				Key:   "uploadpack.advertiseBundleURIs",
-				Value: strconv.FormatBool(enable),
-			},
-		}
+	cfg := gitcmd.ConfigPair{
+		Key:   "uploadpack.advertiseBundleURIs",
+		Value: strconv.FormatBool(enable),
 	}
+
 	if featureflag.BundleURI.IsDisabled(ctx) {
-		return cfg(false)
+		cfg.Value = strconv.FormatBool(false)
 	}
-	return cfg(enable)
+	return []gitcmd.ConfigPair{cfg}
 }
 
 // UploadPackGitConfig return a slice of gitcmd.ConfigPairs you can inject into the
@@ -40,7 +37,7 @@ func CapabilitiesGitConfig(ctx context.Context, enable bool) []gitcmd.ConfigPair
 // who clones/fetches from the repository.
 func UploadPackGitConfig(ctx context.Context, signedURL string) []gitcmd.ConfigPair {
 	if featureflag.BundleURI.IsDisabled(ctx) || signedURL == "" {
-		return []gitcmd.ConfigPair{}
+		return CapabilitiesGitConfig(ctx, false)
 	}
 
 	config := []gitcmd.ConfigPair{