lfsapi: cache the remote list for each endpoint lookup

When we're locking or unlocking multiple times, we may end up looking up
the endpoint a few times.  This is very cheap if we don't need to invoke
Git.  However, as written right now, we call Git every time we validate
a remote, which is wasteful.

To make this more efficient, let's compute the remote list once and
cache it to avoid spawning a process.  Then, we can use the cached value
every time we need to validate a remote.

lfsapi/endpoint_finder.go

@@ -41,6 +41,7 @@ type endpointGitFinder struct {
 	aliasMu     sync.Mutex
 	aliases     map[string]string
 	pushAliases map[string]string
+	remoteList  []string
 
 	accessMu  sync.Mutex
 	urlAccess map[string]creds.AccessMode
@@ -61,6 +62,9 @@ func NewEndpointFinder(ctx lfshttp.Context) EndpointFinder {
 		urlAccess:   make(map[string]creds.AccessMode),
 	}
 
+	remotes, _ := git.RemoteList()
+	e.remoteList = remotes
+
 	e.urlConfig = config.NewURLConfig(e.gitEnv)
 	if v, ok := e.gitEnv.Get("lfs.gitprotocol"); ok {
 		e.gitProtocol = v
@@ -187,7 +191,7 @@ func (e *endpointGitFinder) GitRemoteURL(remote string, forpush bool) string {
 		}
 	}
 
-	if err := git.ValidateRemote(remote); err == nil {
+	if err := git.ValidateRemoteFromList(e.remoteList, remote); err == nil {
 		return remote
 	}