forge: keep whitespaces in commit dialog

Otherwise indentation is lost when commit from erp5_vcs interface

bt5/erp5_forge/SkinTemplateItem/portal_skins/erp5_vcs/BusinessTemplate_viewVcsChangelog/your_changelog.xml

@@ -267,7 +267,7 @@
                 </item>
                 <item>
                     <key> <string>whitespace_preserve</string> </key>
-                    <value> <int>0</int> </value>
+                    <value> <int>1</int> </value>
                 </item>
                 <item>
                     <key> <string>width</string> </key>