Base64: Reformat to match JGit style

Rewrite the initialization of the encoding tables to be more clear,
but slightly slower to setup.  We generally perfer a clear definition
of the data over a slightly slower class load time.

Change-Id: I0c7f89b6ab82dcf71525ffb69a388c312c195913
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>