Allow 'repl :connect' to read args from file

This basically works like in curl, you specify @filename and then the
contents of the file are literally used as if written on the CLI.

@TODO this will not work:
lein repl :connect @./some/file
because there is no realpath resolution
but this works:
lein repl :connect @/full/path
and this as well:
lein repl :connect @some/file

src/leiningen/repl.clj

@@ -52,14 +52,27 @@
     s
     (main/abort "Port is required. See `lein help repl`")))
 
+(defn string-from-file [arg]
+  (if-let [filename-tmp (and (seq arg) (= "@" (subs arg 0 1)) (seq (subs arg 1)))]
+    (let [filename (apply str filename-tmp)
+          errmsg (str "The file '" filename "' can't be read.")]
+      (if-let [content (try (slurp filename)
+                            (catch Exception e
+                            (main/abort errmsg)))]
+        (s/trim content)
+        (main/abort errmsg)))
+      false))
+
 (defn connect-string [project opts]
-  (as-> (str (first opts)) x
-        (s/split x #":")
+  (let [opt (str (first opts))]
+    (if-let [sx (string-from-file opt)]
+      (connect-string project [sx])
+      (as-> (s/split opt #":") x
             (remove s/blank? x)
             (-> (drop-last (count x) [(repl-host project) (client-repl-port project)])
                 (concat x))
             (s/join ":" x)
-        (ensure-port x)))
+            (ensure-port x)))))
 
 (defn options-for-reply [project & {:keys [attach port]}]
   (as-> (:repl-options project) opts

test/leiningen/test/repl.clj

@@ -86,7 +86,14 @@
          ["http://localhost/ham"] {:root "/tmp"}
          ["foo1234"]              {:root "/tmp"}
          []                       {:root "/tmp"}
-         []                       lthelper/with-resources-project))
+         []                       lthelper/with-resources-project)
+    (are [in proj]
+         (is (re-find
+              #"The file '.+' can't be read."
+              (lthelper/abort-msg connect-string proj in)))
+         ["@/tmp/please-do-not-create-this-file-it-will-break-my-test"] {}))
+  (is (= "myhost:23" (connect-string lthelper/sample-project ["@test/sample-connect-string"])))
+  (is (= "http://localhost:23/repl" (connect-string lthelper/sample-project ["@test/sample-connect-string-http"])))
 
   (is (= "127.0.0.1:4242" (connect-string lthelper/sample-project [])))
   (is (= "127.0.0.1:4343" (connect-string lthelper/sample-project ["4343"])))

test/sample-connect-string

@@ -0,0 +1 @@
+myhost:23

test/sample-connect-string-http

@@ -0,0 +1 @@
+http://localhost:23/repl