When changeCleanup.abandonIfMergeable is set to false, the auto abandon
implementation adds "-is:mergeable" to the query that is used to find
changes to be abandoned. When index.change.indexMergeable is disabled,
this causes the query to fail and no changes get auto abandoned.
Fix the ChangeCleanupConfig to disregard abandonIfMergeable when the
indexMergeable is disabled, and log a warning message that the setting
is ineffective.
Extend the documentation of changeCleanup.abandonIfMergeable to mention
this case.
Add test coverage.
Change-Id: Iafe1ef8637deed30b41c48a9f64f199793992870
f4bc2568e9