add git subprocess runner and remote model

Run git directly through Unix.create_process so that user supplied values
such as URLs stay separate argv entries, and model a remote as its fetch
URLs, its effective push destinations and its explicit pushurl entries.

lib/dune

@@ -1,4 +1,4 @@
 (library
  (name git_fleet)
  (public_name git-fleet.lib)
- (libraries cmdliner))
+ (libraries cmdliner unix))