parser: recognise git commit consisting only of empty new file

parser: recognise git commit consisting only of empty new file

Commits with only an empty new file are liable to be missed.
The parser state machine doesn't recognise the headers "new
file mode" and "index": teach it about them.

Add a test to demonstrate.

It's a little bit academic as you don't usually send patches like
that but sometimes you do, especially if you're a snowpatch dev :)

Closes: #256
Reported-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>
(cherry picked from commit 9dd0b5ba843ccac5cf768a1c34226abe8b85cf6d)