Fix error message and clarify git version req; fixes dergachev#15

Author: dergachev

README.md

@@ -17,7 +17,7 @@ Then enable the new GitRemote SCM type in [http://redmine-root/settings?tab=repo
 ![](https://dl.dropbox.com/u/29440342/screenshots/AYKNZDTB-2014.11.27-15-59-06.png)
 
 The plugin shells out to the following binaries, so make sure they're available:
-* git
+* git 1.7.5+ - a version recent enough to support `get remote add --mirror=fetch origin URL`
 * ssh-keyscan
 * ssh-keygen
 

app/models/repository/git_remote.rb

@@ -110,7 +110,7 @@ def ensure_possibly_empty_clone_exists
       end
 
       unless system "git", "--git-dir", clone_path, "remote", "add", "--mirror=fetch", "origin",  clone_url
-        return  "Unable to run: git --git-dir #{clone_path} remote add #{clone_url}"
+        return  "Unable to run: git --git-dir #{clone_path} remote add --mirror=fetch origin #{clone_url}"
       end
     end
   end