Print git commit when running Bundler from a git checkout

[indirect]

When running bundler directly from a git checkout (for example via the dbundle alias suggested by DEVELOPMENT.md), it would be SUPER helpful to print the git commit out when running dbundle --version, and also when running bundle env.

[segiddins]

diff --git a/lib/bundler/version.rb b/lib/bundler/version.rb
index 3f9a1211f..909d80964 100644
--- a/lib/bundler/version.rb
+++ b/lib/bundler/version.rb
@@ -7,7 +7,17 @@ module Bundler
   # We're doing this because we might write tests that deal
   # with other versions of bundler and we are unsure how to
   # handle this better.
-  VERSION = "1.15.1" unless defined?(::Bundler::VERSION)
+  unless defined?(::Bundler::VERSION)
+    require "bundler/constants"
+    git = File.expand_path("../../../.git", __FILE__)
+    if File.directory?(git)
+      Dir.chdir(git) do
+        git_describe = `git describe --always 2>#{NULL}`.strip.sub(/^v/, "")
+        VERSION = git_describe unless git_describe.empty?
+      end
+    end
+    VERSION = "1.15.1" unless defined?(::Bundler::VERSION)
+  end
 
   def self.overwrite_loaded_gem_version
     begin