Set commit ID in result of the /changes/*/revisions/*/commit endpoint

Setting the commit ID is important when a revision ID other than a
commit ID is used, e.g. a legacy numeric patch set number, or the
literal 'current'.

Also explain in the documentation in which case the commit ID is not
set.

Change-Id: Iadcff2c7cbe98be8c826a0ea6d0aa8bd8ff2a6be
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/server/change/GetCommit.java

@@ -54,6 +54,7 @@ public class GetCommit implements RestReadView<RevisionResource> {
       rw.parseBody(commit);
       CommitInfo info = json.create(ChangeJson.NO_OPTIONS)
           .toCommit(rsrc.getControl(), rw, commit, addLinks);
+      info.commit = commit.name();
       Response<CommitInfo> r = Response.ok(info);
       if (rsrc.isCacheable()) {
         r.caching(CacheControl.PRIVATE(7, TimeUnit.DAYS));