Add support for replacing only the diff of a particular patch

[NonLogicalDev]

There are cases where I would like to stg import a GNU patch, but would like to update an existing patch rather than creating a new one.

Currently the closest I managed to get to this is

stg edit -d -f diff.patch PATCH_NAME

However this replaces the contents of the patch with the diff overwriting the message. Would be nice to be able to replace just the diff portion of the patch.

Update:

I have attempted to implement this. Initially I tried to wedge this into edit command, but I feel like a new command is a better approach for this one. So far I have so far opted for this:

stg replace [ -p PATCH_NAME ] [ --import-description ] diff.patch