Support multiple %base arguments with refs/for/

Multiple %base arguments are necessary when pushing a merge commit
in order to mark the uninteresting part of the TreeWalk for each
parent line.

Example:
  $ git push origin HEAD:refs/for/master%base=commitId1,base=commitId2

Change-Id: Ie291e7eb2d5755d7c69b5625662af03561de1386

Documentation/user-upload.txt

@@ -366,6 +366,15 @@ by setting the merge base SHA-1 using the '%base' argument:
   git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/master%base=$(git rev-parse origin/master)
 ====
 
+It is also possible to specify more than one '%base' argument.
+This may be useful when pushing a merge commit. Note that the '%'
+character has only to be provided once, for the first '%base'
+argument:
+
+====
+  git push ssh://john.doe@git.example.com:29418/kernel/common HEAD:refs/for/master%base=commit-id1,base=commit-id2
+====
+
 
 repo upload
 -----------

gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java

@@ -981,10 +981,10 @@ public class ReceiveCommits {
     RefControl ctl;
     Set<Account.Id> reviewer = Sets.newLinkedHashSet();
     Set<Account.Id> cc = Sets.newLinkedHashSet();
-    RevCommit baseCommit;
+    List<RevCommit> baseCommit;
 
     @Option(name = "--base", metaVar = "BASE", usage = "merge base of changes")
-    ObjectId base;
+    List<ObjectId> base;
 
     @Option(name = "--topic", metaVar = "NAME", usage = "attach topic to changes")
     String topic;
@@ -1135,20 +1135,24 @@ public class ReceiveCommits {
 
     RevWalk walk = rp.getRevWalk();
     if (magicBranch.base != null) {
-      try {
+      magicBranch.baseCommit = Lists.newArrayListWithCapacity(
-        magicBranch.baseCommit = walk.parseCommit(magicBranch.base);
+          magicBranch.base.size());
-      } catch (IncorrectObjectTypeException notCommit) {
+      for (ObjectId id : magicBranch.base) {
-        reject(cmd, "base must be a commit");
+        try {
-        return;
+          magicBranch.baseCommit.add(walk.parseCommit(id));
-      } catch (MissingObjectException e) {
+        } catch (IncorrectObjectTypeException notCommit) {
-        reject(cmd, "base not found");
+          reject(cmd, "base must be a commit");
-        return;
+          return;
-      } catch (IOException e) {
+        } catch (MissingObjectException e) {
-        log.warn(String.format(
+          reject(cmd, "base not found");
-            "Project %s cannot read %s",
+          return;
-            project.getName(), magicBranch.base.name()), e);
+        } catch (IOException e) {
-        reject(cmd, "internal server error");
+          log.warn(String.format(
-        return;
+              "Project %s cannot read %s",
+              project.getName(), id.name()), e);
+          reject(cmd, "internal server error");
+          return;
+        }
       }
     }
 
@@ -1288,7 +1292,9 @@ public class ReceiveCommits {
       Set<ObjectId> existing = Sets.newHashSet();
       walk.markStart(walk.parseCommit(magicBranch.cmd.getNewId()));
       if (magicBranch.baseCommit != null) {
-        walk.markUninteresting(magicBranch.baseCommit);
+        for (RevCommit c : magicBranch.baseCommit) {
+          walk.markUninteresting(c);
+        }
         assert magicBranch.ctl != null;
         Ref targetRef = allRefs.get(magicBranch.ctl.getRefName());
         if (targetRef != null) {