Make stockroom work in a non-git workflow

[hhsecond]

Description

Stockroom currently works only if the root is a git repository. While this is a good practice, there are cases where git might not be part of the workflow and stockroom could help in storing models.

• It should be able to read the commit from user arguments instead of from head.stock
• It should not be writing to the head.stock file
• Maybe it is a good idea to control this behavior while stock init