Upgrade old StGIT branches to new-format metadata

There used to be a "stg branch --convert" command that switched
between "old" and "new" format metadata. But my recent metadata
cleanup patches introduced a "new new" format, and more are hopefully
on the way, so it was time to start versioning the metadata format
explicitly in order to avoid future headaches.

This patch removes the "stg branch --convert" command, and makes StGIT
automatically upgrade older formats to the latest format. It stores
the format (as an integer) in the config file. The current metadata
format version number is 2 (the "old" format is 0, and the "new"
format is 1).

Signed-off-by: Karl Hasselström <kha@treskal.com>