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/git.mli

@@ -0,0 +1,64 @@
+(** Execution of [git] subprocesses and inspection of repository remotes.
+
+    Every operation runs the [git] executable directly. User supplied values,
+    such as URLs, are always passed as separate arguments and are never
+    interpolated into a shell command line. *)
+
+(** Failures that are expected during normal use of the command line. *)
+type error =
+  | Git_unavailable of string
+      (** The [git] executable could not be started. *)
+  | Not_a_repository of string  (** The directory is not a Git work tree. *)
+  | No_such_remote of string  (** The repository has no such remote. *)
+  | Not_a_push_destination of { remote : string; url : string }
+      (** The URL is not configured as a push destination of the remote. *)
+  | Implicit_push_destination of { remote : string; url : string }
+      (** The URL is the remote's implicit push destination, which follows its
+          fetch URL and so cannot be removed on its own. *)
+  | Command_failed of { args : string list; status : int; stderr : string }
+      (** A [git] command exited with a non-zero status. *)
+
+val string_of_error : error -> string
+(** A concise single line description, without a trailing newline. *)
+
+(** A Git remote.
+
+    [push_urls] are the destinations Git actually pushes to. When a remote has
+    no [remote.<name>.pushurl] entries, Git pushes to the fetch URL, so
+    [explicit_push_urls] is empty while [push_urls] is not. *)
+type remote = {
+  name : string;
+  fetch_urls : string list;
+  push_urls : string list;
+  explicit_push_urls : string list;
+}
+
+val is_repository : cwd:string -> bool
+(** Whether [cwd] is inside a Git work tree. *)
+
+val remotes : cwd:string -> (string list, error) result
+(** The names of every configured remote, in Git's order. *)
+
+val remote_exists : cwd:string -> name:string -> (bool, error) result
+val get_fetch_urls : cwd:string -> name:string -> (string list, error) result
+
+val get_push_urls : cwd:string -> name:string -> (string list, error) result
+(** The effective push destinations, including the implicit one. *)
+
+val get_explicit_push_urls :
+  cwd:string -> name:string -> (string list, error) result
+(** The [remote.<name>.pushurl] entries, in configuration order. *)
+
+val remote : cwd:string -> name:string -> (remote, error) result
+
+(** A single change to a repository's configuration. *)
+type action =
+  | Add_remote of { name : string; url : string }
+  | Set_fetch_url of { name : string; url : string }
+  | Add_push_url of { name : string; url : string }
+  | Clear_push_urls of { name : string }
+
+val command_of_action : action -> string list
+(** The equivalent Git command line, as an argument list starting with ["git"]. *)
+
+val apply : cwd:string -> action -> (unit, error) result