Option to reject implicit merges when pushing changes for review

An implicit merge is a case where by submitting an open change one also
merges a branch into the target branch. Typically, this happens when a
change is done on top of master and, by mistake, pushed to stable
branch. Merging this change would also implicitly merge master into
stable.

Example 1:

  o < change pushed for stable
  |
  o < master
  |
  o < stable

  Submitting this change will implicitly merge master into stable:

  o < change pushed for stable, stable
  |
  o < master
  |
  o

Example 2:

           o < change pushed for stable
           |
  master > o   o < stable
            \ /
             o

  Submitting this change will implicitly merge master into stable:

               o < stable
              /|
            /  |
           o < change pushed for stable
           |   |
  master > o   o
            \ /
             o

A new project property receive.rejectImplicitMerges controls whether an
implicit merge will be rejected. When an implicit merge is detected
Gerrit will print error(s) to the user:

  remote: ERROR: Implicit Merge of 39adddb Commit message subject
  remote: ERROR: Implicit Merge of ...

and will reject the push.

Bug: issue 1107
Change-Id: I0b14c64bebe28ea5579fc11f6beedacf5982e5aa

gerrit-reviewdb/src/main/java/com/google/gerrit/reviewdb/client/Project.java

@@ -99,6 +99,8 @@ public final class Project {
   protected InheritableBoolean enableSignedPush;
   protected InheritableBoolean requireSignedPush;
 
+  protected InheritableBoolean rejectImplicitMerges;
+
   protected Project() {
   }
 
@@ -151,6 +153,10 @@ public final class Project {
     return maxObjectSizeLimit;
   }
 
+  public InheritableBoolean getRejectImplicitMerges() {
+    return rejectImplicitMerges;
+  }
+
   public void setUseContributorAgreements(final InheritableBoolean u) {
     useContributorAgreements = u;
   }
@@ -196,6 +202,10 @@ public final class Project {
     maxObjectSizeLimit = limit;
   }
 
+  public void setRejectImplicitMerges(InheritableBoolean check) {
+    rejectImplicitMerges = check;
+  }
+
   public SubmitType getSubmitType() {
     return submitType;
   }