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>

Documentation/rest-api-changes.txt

@@ -2235,6 +2235,7 @@ describes the revision.
 
   )]}'
   {
+    "commit": "674ac754f91e64a0efb8087e59a176484bd534d1",
     "parents": [
       {
         "commit": "1eee2c9d8f352483781e772f35dc586a69ff5646",
@@ -3947,7 +3948,10 @@ The `CommitInfo` entity contains information about a commit.
 [options="header",cols="1,^1,5"]
 |===========================
 |Field Name    ||Description
-|`commit`      ||The commit ID.
+|`commit`      |Optional|
+The commit ID. Not set if included in a link:#revision-info[
+RevisionInfo] entity that is contained in a map which has the commit ID
+as key.
 |`parents`     ||
 The parent commits of this commit as a list of
 link:#commit-info[CommitInfo] entities. In each parent

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));