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-gwtui/src/main/java/com/google/gerrit/client/admin/AdminConstants.java

@@ -45,6 +45,7 @@ public interface AdminConstants extends Constants {
   String enableSignedPush();
   String requireSignedPush();
   String requireChangeID();
+  String rejectImplicitMerges();
   String headingMaxObjectSizeLimit();
   String headingGroupOptions();
   String isVisibleToAll();