Issue entityreference autocomplete runs too many too large queries

When starting to type a node id or issue title in the Issue relations field on an issue, autocomplete tries to be helpful and provide autocomplete suggestions for what the user has typed. Unfortunately, due to the very large number of (issue) nodes on drupal.org this means that until the user has typed a handful of characters, the match will return a result set that exceeds PHP's memory_limit in size. That means the autocomplete fails with an AJAX error and the background process that runs the (expensive, huge) query gets killed:

An AJAX HTTP error occurred.
HTTP Result Code: 200
Debugging information follows.
Path: https://drupal.org/entityreference/autocomplete/single/field_issue_parent/node/project_issue/NULL
StatusText: OK
ResponseText: Fatal error: Allowed memory size of 419430040 bytes exhausted (tried to allocate 77 bytes) in /var/www/drupal.org/htdocs/sites/all/modules/project_issue/plugins/entityreference_selection/ProjectIssue_SelectionHandler_Issues.class.php on line 123

This can't be good for d.o performance. Even if the result set fits in memory, it quite possibly returns (tens of?) thousands of nid|node rows that then need to get rendered in the browser... and it'll be making the MySQL server query cache cry.

To make this less of an issue, I suggest a few tweaks to the autocomplete handler:

• Do not perform a title query until the user has typed at least three characters, or possibly even a few more.
• Add a limit clause to the query so the result set can never be gigantic.
• Order the result set by relevance or modification date. (I expect we have no relevance field, but issue status maybe? Closed filters to bottom? Length of matched title?)

Apropos of nothing, the query builder isn't using db_like() and for minimal speed gain its substr() call can be replaced with strpos(), which is 50% faster at checking the first character of a string, it seems ;-)