Skip git revision in pom.xml if empty git repo.

Tests if the git ref path exists before slurping the SHA from it. If it does not
exist, return nil instead of throwing an error. Also remove empty references to
the SHA where they may occur.

src/leiningen/pom.clj

@@ -29,12 +29,16 @@
       git-dir-file)))
 
 (defn- read-git-ref
-  "Reads the commit SHA1 for a git ref path."
+  "Reads the commit SHA1 for a git ref path, or nil if no commit exist."
   [git-dir ref-path]
-  (.trim (slurp (str (io/file git-dir ref-path)))))
+  (let [ref (io/file git-dir ref-path)]
+    (if (.exists ref)
+      (.trim (slurp ref))
+      nil)))
 
 (defn- read-git-head
-  "Reads the value of HEAD and returns a commit SHA1."
+  "Reads the value of HEAD and returns a commit SHA1, or nil if no commit
+  exist."
   [git-dir]
   (let [head (.trim (slurp (str (io/file git-dir "HEAD"))))]
     (if-let [ref-path (second (re-find #"ref: (\S+)" head))]
@@ -78,7 +82,8 @@
             [:connection (str "scm:git:" (:public-clone urls))])
        (and (:dev-clone urls)
             [:developerConnection (str "scm:git:" (:dev-clone urls))])
-       [:tag head]
+       (and head
+            [:tag head])
        [:url (:browse urls)]])
     (catch java.io.FileNotFoundException _)))
 
@@ -340,7 +345,8 @@
                        (.setProperty "artifactId" (:name project)))
           git-head (resolve-git-dir project)]
       (when (.exists git-head)
-        (.setProperty properties "revision" (read-git-head git-head)))
+        (if-let [revision (read-git-head git-head)]
+          (.setProperty properties "revision" revision)))
       (.store properties baos "Leiningen"))
     (str baos)))