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-server/src/main/java/com/google/gerrit/server/project/PutConfig.java

@@ -145,6 +145,10 @@ public class PutConfig implements RestModifyView<ProjectResource, ConfigInput> {
         }
       }
 
+      if (input.rejectImplicitMerges != null) {
+        p.setRejectImplicitMerges(input.rejectImplicitMerges);
+      }
+
       if (input.maxObjectSizeLimit != null) {
         p.setMaxObjectSizeLimit(input.maxObjectSizeLimit);
       }