Add function el-get-plist-get-with-default

It encapsulates the following common pattern, used several times in
el-get to allow recipe properties to override defaults if present:

(if (plist-member plist prop)
    (plist-get plist prop)
  default)

el-get-core.el

@@ -335,7 +335,7 @@ makes it easier to conditionally splice a command into the list.
                (args    (if shell
 			    (mapcar #'shell-quote-argument (plist-get c :args))
 			  (plist-get c :args)))
-               (sync    (if (plist-member c :sync) (plist-get c :sync)
+               (sync    (el-get-plist-get-with-default c :sync
                           el-get-default-process-sync))
 	       (stdin   (plist-get c :stdin))
                (default-directory (if cdir
@@ -430,4 +430,16 @@ out if it's nil."
 	   name))
 	command)))))
 
+(defun el-get-plist-get-with-default (plist prop def)
+  "Same as (plist-get PLIST PROP), but falls back to DEF.
+
+Specifically, if (plist-member PLIST PROP) is nil, then returns
+DEF instead. Note that having a property set to nil is not the
+same as having it unset."
+  (if (plist-member plist prop)
+      (plist-get plist prop)
+    def))
+(put 'el-get-plist-get-with-default 'lisp-indent-function
+     (get 'prog2 'lisp-indent-function))
+
 (provide 'el-get-core)

methods/el-get-git.el

@@ -51,9 +51,8 @@ found."
                                     (not submodule-prop)))
          (checkout (or (plist-get source :checkout)
 		       (plist-get source :checksum)))
-         (shallow (if (plist-member source :shallow)
-                      (plist-get source :shallow)
-                    el-get-git-shallow-clone))
+         (shallow (el-get-plist-get-with-default source :shallow
+                                                 el-get-git-shallow-clone))
 	 (clone-args (append '("--no-pager" "clone")
                              (when shallow '("--depth" "1"))
 			     (cond