RepoCommand.RemoteReader: Add method to read contents and mode of file

RepoCommand.RemoteReader: Add method to read contents and mode of file

The RepoCommand.RemoteReader interface doesn't offer access to the mode
of a file. Caller can only default to mark the copied objects as regular
files, losing e.g. the executable bit (if set).

Add a new method readFileWithMode that returns the contents and mode of
the remote file. It supersedes the readFile method, that is marked as
deprecated.

Now callers can set correctly the file mode of the copied file.

Change-Id: I8fce01e4bc5707434c0cbc4aebbae1b6b64756f0
Signed-off-by: Ivan Frade <ifrade@google.com>