dirstate-v2: Add `hg debugupgraderepo` command support

Authored by SimonSapin.


dirstate-v2: Add hg debugupgraderepo command support

This command changes changes the file formats used inside an existing
repository to what they would be in a new repository with the current config.

For example:

hg debugupgraderepo --config format.exp-dirstate-v2=1 --run
hg debugupgraderepo --config format.exp-dirstate-v2=0 --run

If a repository has a dirstate in v1 format, the first command would upgrade it
to dirstate-v2. Conversely, if a repository has a dirstate in v2 format, the
second command would downgrade it to v1. (Both may also run some unrelated

Since format.exp-dirstate-v2 is currently disabled by default, not specifying
it in --config or any configuration file would result in the second command.

Differential Revision: https://phab.mercurial-scm.org/D10769