Add "compare" command.

This command allows you to view a compare view in the browser
from a START to an END commit.

Example:
  hub compare AWESOME_PATCH...MASTER

README.md

@@ -165,6 +165,20 @@ login" below for details.
     $ git browse -p resque
     > open https://github.com:YOUR_USER/resque
 
+### git compare
+
+    $ git compare refactor
+    > open http://github.com/CURRENT_REPO/compare/refactor
+
+    $ git compare 1.0...1.1
+    > open http://github.com/CURRENT_REPO/compare/1.0...1.1
+
+    $ git compare -u fix
+    > (http://github.com/CURRENT_REPO/compare/fix)
+
+    $ git compare myfork patch
+    > open http://github.com/myfork/compare/patch
+
 ### git help
 
     $ git help

lib/hub/commands.rb

@@ -232,6 +232,36 @@ def browse(args)
       args.push "#{protocol}://github.com/#{user}/#{repo}"
     end
 
+    # $ hub compare 1.0...fix
+    # > open http://github.com/CURRENT_REPO/compare/1.0...fix
+    # $ hub compare refactor
+    # > open http://github.com/CURRENT_REPO/comapre/refactor
+    # $ hub compare myfork feature
+    # > open http://github.com/myfork/REPO/compare/feature
+    # $ hub compare -u 1.0...2.0
+    # (Prints the URL for the compare view)
+    def compare(args)
+        args.shift
+        urlOnly = args.delete('-u') ? true : false
+        range = args.pop
+        user = args.pop || OWNER
+
+        if range and !OWNER.empty?
+            url = "http://github.com/#{user}/#{REPO}/compare/#{range}"
+        else
+            warn "Usage: hub compare [<START>...]<END>"
+            exit(1)
+        end
+
+        if urlOnly
+            puts url
+            exit
+        else
+            args.executable = ENV['BROWSER'] || 'open'
+            args.push url
+        end
+    end
+
     # $ hub hub standalone
     # Prints the "standalone" version of hub for an easy, memorable
     # installation sequence:

man/hub.1.ron

@@ -58,6 +58,12 @@ alias command displays information on configuring your environment:
     page with https. If the repository isn't specified, `browse` opens
     the page of the repository found in the current directory.
 
+  * `git compare` [`-u`] [<USER>] [<START>...]<END>:  
+    Open a Github compare view page in the system's default web browser.
+    <START> to <END> are branch names, tag names, or commit SHA1s specifying
+    the range of history to compare. If <START> is omitted,
+    the repository's default branch is assumed.
+
   * `git submodule add` [`-p`] <OPTIONS> [<USER>/]<REPOSITORY> <DIRECTORY>:  
     Submodule repository "git://github.com/<USER>/<REPOSITORY>.git" into
     <DIRECTORY> as with git-submodule(1). When <USER>/ is omitted, assumes
@@ -151,6 +157,20 @@ cloning:
     $ git browse -p resque
     > open https://github.com:YOUR_USER/resque
 
+### git compare
+
+    $ git compare refactor
+    > open http://github.com/CURRENT_REPO/compare/refactor
+
+    $ git compare 1.0...1.1
+    > open http://github.com/CURRENT_REPO/compare/1.0...1.1
+
+    $ git compare -u fix
+    > (http://github.com/CURRENT_REPO/compare/fix)
+
+    $ git compare myfork patch
+    > open http://github.com/myfork/compare/patch
+
 ### git help
 
     $ git help