Make the default encoding when reading commits UTF-8

When reading commits the system default encoding was used if no
encoding was specified in the commit. The patch modifies the test
to add a check that commit message was encoded correctly (the
test fails on old implementation if system encoding is not UTF-8)
and fixes Commit.decode() method to use UTF-8 if encoding is not
specified in the commit object.

Change-Id: I27101da3c2eb6edd0c4a9e4c0523e48b286e3cd5
Signed-off-by: Constantine Plotnikov <constantine.plotnikov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>