git: allow validating remote from a cached list

Right now, every time we call `ValidateRemote`, we look up the list of
remotes by invoking `git remote`.  It would be nice if we could cache
the list of remotes so we could validate the remote multiple times
without the expense of calling Git multiple times, so let's add a
function, `ValidateRemoteFromList`, which can take a cached list and
perform the same computation.

git/git.go

@@ -531,13 +531,20 @@ func ValidateRemote(remote string) error {
 	if err != nil {
 		return err
 	}
+	return ValidateRemoteFromList(remotes, remote)
+}
+
+// ValidateRemote checks that a named remote is valid for use given a list from
+// RemoteList.  This is completely identical to ValidateRemote, except that it
+// allows caching the remote list.
+func ValidateRemoteFromList(remotes []string, remote string) error {
 	for _, r := range remotes {
 		if r == remote {
 			return nil
 		}
 	}
 
-	if err = ValidateRemoteURL(remote); err == nil {
+	if err := ValidateRemoteURL(remote); err == nil {
 		return nil
 	}