Add support for post-dcommit hooks

git_cl/test/post-dcommit-hook-test.sh

+#!/bin/bash
+
+set -e
+
+. ./test-lib.sh
+
+setup_initsvn
+setup_gitsvn
+
+(
+  set -e
+  cd git-svn
+
+  cat > .git/hooks/post-cl-dcommit << _EOF
+#!/bin/bash
+git branch -m COMMITTED
+_EOF
+  chmod +x .git/hooks/post-cl-dcommit
+
+  git config rietveld.server localhost:1
+  git checkout -q --track -b work
+  echo "some work done" >> test
+  git add test; git commit -q -m "work"
+
+  test_expect_success "dcommitted code" \
+      "$GIT_CL dcommit -f --tbr --bypass-hooks -m 'dcommit'"
+
+  test_expect_success "post-cl-dcommit hook executed" \
+      "git symbolic-ref HEAD | grep -q COMMITTED"
+)
+SUCCESS=$?
+
+cleanup
+
+if [ $SUCCESS == 0 ]; then
+  echo PASS
+fi