Fix updating existing custom branch

[pohy]

1. The git rev-parse command was failing because of unspecified remote

/usr/bin/git rev-parse --verify codebase-diagram
fatal: Needed a single revision
Branch codebase-diagram does not yet exist, creating codebase-diagram.

2. Checking out the already existing custom branch would fail because of modified working tree.

/usr/bin/git checkout codebase-diagram
error: The following untracked working tree files would be overwritten by checkout:
	codebase-diagram.svg
Please move or remove them before you switch branches.
Aborting

Fixes #42