bundle2: prefix all bundling debug messages with 'bundle2-ouput:' This makes the origin of the message more explicit.

bundle2: introduce a specific function for bundling debug message The bundling process is very verbose, we would like to be able to hide such output behind a configuration flag and have it more explicitly referencing bundle2. The first step is to gather all these messages in a dedicated function. The same gathering will be later do for debug message issue by unbundling.

parsers: simplify the code computing the phases We recently simplified the code computing the heads of a repo. This patch uses the same simplification for phase computation. We use index_get_parents instead of replicating its code.