Mirage support

• Fix wrong interaction of in-memory views and temporary branches in the store
  (#237)
• Fix Irmin.update_tag for HTTP clients
• Initial MirageOS support. Expose Mirage_irmin.KV_RO to surface an
  Irmin store as a read-only key/value store implementing `V1_LWT.KV_RO
  (#107)
• Expose `Irmin_git.Memory_ext. This allows the Git memory backend to be
  configured with a non-empty conduit context.
• Expose Irmin.SYNC
• Transmit client tasks to the HTTP server on DELETE too (#227, @dsheets)
• Do note expose private types in the public interface (#234, @koleini)
• Fix missing zero padding for date pretty-printing (#228, @dsheets)
• Update the tests to use ocaml-git.1.6.0
• Improve the style of the HTTP commit graph.
• Constraint the string tags to contain only alpha-numeric characters
  and few mores (-, _, '.' and /) (#186)
• Fix a race condition in Irmin.clone. (#221)
• Escpate double quotes in the output of commit messages to workaround
  HTML display issues. (#222)