Fix problem with missing libs/sync/test target.

The problem can be reproduced in a clean regression
test directory by first running tests with --tag=master
and then running tests with --tag=develop.  The fix,
as Beman pointed out, is to init the submodules.
Fortunately the init command is a noop for existing
submodules so added this command to the git testing
sequence when the boost_root directory already exists.

With this patch applied, I am now able to run tests
with master first, and develop second, no error. Tested
on Linux and Darwin.

tools/regression/src/regression.py

@@ -907,6 +907,7 @@ class runner:
                 self.git_command( 'remote', 'set-branches', '--add', 'origin',
                     branch)
                 self.git_command( 'pull', '--recurse-submodules' )
+                self.git_command( 'submodule', 'init')
                 self.git_command( 'submodule', 'update')
                 self.git_command( 'checkout', branch)
                 if clean: