Optimisation: Added an index on bug_id field in the mantis_bug_file_table.

A admin/upgrades/0_18_inc.php

• Added to include upgrades since 0.18.0

M admin/upgrade.php

• Add upgrade steps in 0_18_inc.php

M core/database_api.php

• (db_key_exists_on_field): Checks if there exists a field on the specified table/field of the specified type.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2291 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9