Add support for relative to base scope command line variable overrides

Currently, if we say:

$ b dir/ ./foo=bar

The scope the foo=bar is set on is relative to CWD, not dir/. While this
may seem wrong at first, this is the least surprising behavior when we
take into account that there can be multiple dir/'s.

Sometimes, however, we do want the override directory to be treated relative
to (every) target's base scope that we are building. To support this we are
extending the '.' and '..' special directory names (which are still resolved
relative to CWD) with '...', which means "relative to the base scope of every
target in the buildspec". For example:

$ b dir/ .../foo=bar

Is equivalent to:

$ b dir/ dir/foo=bar

And:

$ b liba/ libb/ .../tests/foo=bar

Is equivalent to:

$ b liba/ libb/ liba/tests/foo=bar libb/tests/foo=bar

1 files changed, 2 insertions, 2 deletions

diff --git a/build2/file.cxx b/build2/file.cxx
index 05527f3..3f0e7b7 100644
--- a/build2/file.cxx
+++ b/build2/file.cxx
@@ -221,7 +221,7 @@ namespace build2
   scope_map::iterator
   create_root (scope& l, const dir_path& out_root, const dir_path& src_root)
   {
-    auto i (scopes.rw (l).insert (out_root, true));
+    auto i (scopes.rw (l).insert (out_root, true /* root */));
     scope& rs (i->second);
 
     // Set out_path. Note that src_path is set in setup_root() below.
@@ -348,7 +348,7 @@ namespace build2
     // First, enter the scope into the map and see if it is in any project. If
     // it is not, then there is nothing else to do.
     //
-    auto i (scopes.rw (root).insert (p, false));
+    auto i (scopes.rw (root).insert (p));
     scope& base (i->second);
     scope* rs (base.root_scope ());