Add context to content commit

Content commit is updated to take in a context, allowing
content to be committed within the same context the writer
was in. This is useful when commit may be able to use more
context to complete the action rather than creating its own.
An example of this being useful is for the metadata implementation
of content, having a context allows tests to fully create
content in one database transaction by making use of the context.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

differ/differ.go

@@ -179,7 +179,7 @@ func (s *walkingDiff) DiffMounts(ctx context.Context, lower, upper []mount.Mount
 	}
 
 	dgst := cw.Digest()
-	if err := cw.Commit(0, dgst, opts...); err != nil {
+	if err := cw.Commit(ctx, 0, dgst, opts...); err != nil {
 		return emptyDesc, errors.Wrap(err, "failed to commit")
 	}