Add a -p/--for-release-of-packages command line argument

`-p pkgs` is the equivalent of `--root . --only-packages pkgs`

bin/main.ml

@@ -110,7 +110,6 @@ let help_secs =
 let common =
   let make
         concurrency
-        only_packages
         debug_rules
         debug_actions
         debug_dep_path
@@ -118,7 +117,7 @@ let common =
         dev_mode
         verbose
         workspace_file
-        root
+        (root, only_packages)
     =
     let root, to_cwd =
       match root with
@@ -213,9 +212,38 @@ let common =
                     targets given on the command line. It is only intended
                     for scripts.|})
   in
+  let for_release = "for-release-of-packages" in
+  let frop =
+    Arg.(value
+         & opt (some string) None
+         & info ["p"; for_release] ~docs ~docv:"PACKAGES"
+             ~doc:{|Shorthand for $(b,--root . --only-packages PACKAGE).|})
+  in
+  let root_and_only_packages =
+    let merge root only_packages release =
+      match release, root, only_packages with
+      | Some _, Some _, _ ->
+        `Error (true,
+                sprintf
+                  "Cannot use %s and --root simultaneously"
+                  for_release)
+      | Some _, _, Some _ ->
+        `Error (true,
+                sprintf
+                  "Cannot use %s and --only-packages simultaneously"
+                  for_release)
+      | Some pkgs, None, None ->
+        `Ok (Some ".", Some pkgs)
+      | None, _, _ ->
+        `Ok (root, only_packages)
+    in
+    Term.(ret (const merge
+               $ root
+               $ only_packages
+               $ frop))
+  in
   Term.(const make
         $ concurrency
-        $ only_packages
         $ drules
         $ dactions
         $ ddep_path
@@ -223,7 +251,7 @@ let common =
         $ dev
         $ verbose
         $ workspace_file
-        $ root
+        $ root_and_only_packages
        )
 
 let installed_libraries =

doc/manual.org

@@ -1178,9 +1178,12 @@ You should set the =build:= field of your =<package>.opam= file as
 follows:
 
 #+begin_src
-build: [["jbuilder" "build" "--only-packages" "<package>" "--root" "." "-j" jobs "@install"]]
+build: [["jbuilder" "build" "-p" "<package>" "-j" jobs]]
 #+end_src
 
+=-p pkg= is a shorthand for =--root . --only-packages pkg=. =-p= is
+the short version of =--for-release-of-packages=.
+
 This has the following effects:
 - it tells jbuilder to build everything that is installable and to
   ignore packages other than =<package>= defined in your project

src/main.ml

@@ -24,7 +24,7 @@ let setup ?(log=Log.no_log) ?filter_out_optional_stanzas_with_missing_deps
     String_set.iter set ~f:(fun pkg ->
       if not (String_map.mem pkg conf.packages) then
         die "@{<error>Error@}: I don't know about package %s \
-             (passed through --only-packages)%s"
+             (passed through --only-packages/--release)%s"
           pkg (hint pkg (String_map.keys conf.packages))));
   let workspace =
     match workspace with