Use page.sha instead of a content-based hash for collision detection.

lib/gollum/app.rb

@@ -176,7 +176,7 @@ module Precious
               @page         = page
               @page.version = wiki.repo.log(wiki.ref, @page.path).first
               @content      = page.text_data
-              @etag       = Digest::SHA1.hexdigest(@content)
+              @etag         = page.sha
               mustache :edit
           else
             redirect_to("/create/#{encodeURIComponent(@name)}")
@@ -286,7 +286,7 @@ module Precious
         page      = wiki.paged(page_name, path, exact = true)
         
         return if page.nil?
-        if etag != Digest::SHA1.hexdigest(page.text_data) 
+        if etag != page.sha
           halt 412, 'For future use: some helpful data for resolving the conflict here.'
         end