Implement REST endpoint to reindex a project

We recently added a project index to Gerrit. This complements the
existing set of indices (change, account, group). For this new index we
want to offer a REST endpoint as well to reindex a project with the
option to recusively reindex the children as well.

Unfortunately, we previously bound /index to the endpoint that reindexes
all changes in the project. This commit changes that binding to
/index.changes.
We make an effort to keep the REST API stable, but this is just an
administrative endpoint and it seems to be a better fit to just
repurpose the endpoint name.

This commit adds tests and methods for the Java extension API.

Change-Id: I2b3bb9f38c94938b98059da193e80289996a4a41
diff --git a/Documentation/cmd-index.txt b/Documentation/cmd-index.txt
index f535281..2362401 100644
--- a/Documentation/cmd-index.txt
+++ b/Documentation/cmd-index.txt
@@ -136,7 +136,7 @@
 link:cmd-index-changes.html[gerrit index changes]::
 	Index one or more changes.
 
-link:cmd-index-project.html[gerrit index project]::
+link:cmd-index-changes-in-project.html[gerrit index changes-in-project]::
 	Index all the changes in one or more projects.
 
 link:cmd-logging-ls-level.html[gerrit logging ls-level]::