diff fails on empty repository

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System Information

Version:  v0.2.0-alpha-modified

System: linux/amd64

Go:  go1.14.2

Source Revision:  16abc7be5a5ad17a0aeb0f1a7815f5e13a67bd1b

## What did you do?

``` bash
baronial init
mkdir accounts/checking
mkdir -p budget/food/grocery
baronial credit 10.89 ./accounts/checking/ ./budget/food/grocery/
baronial diff
```

## What did you expect to happen?

A summary of the 10.89 being added to the checking account and grocery budget should have been printed.

## What actually happened?

An error message was spat out, complaining that 0000000000000000000000000000000000000000.json doesn't exist.

Stack Trace

ERRO[0000] open /home/marstr/.tmp/tmp.7w3FkcD1mq/.baronial/objects/0000000000000000000000000000000000000000.json: no such file or directory 

Error: exec: not started

Usage:

  baronial diff [refspec] [refspec] [flags]


Flags:

  -h, --help   help for diff

exec: not started

Start by reproducing the reported sequence with `baronial init`, the account and budget directories, `baronial credit`, and `baronial diff`. Trace the `baronial diff` entry point from the missing all-zero object error; done means an empty repository produces the expected 10.89 summary instead of failing.

Written by the indexing model from the issue text.