Fix :syn-include usage of Vim filetype.

Here’s what the Vim manual says in *:syn-include*:

    :sy[ntax] include [@{grouplist-name}] {file-name}

	All syntax items declared in the included file will have the
	"contained" flag added.  In addition, if a group list is
	specified, all top-level syntax items in the included file will
	be added to that list.

We had two rules for `rustModPath`, one `contained` and the other not.
The effect was that the second (now renamed to `rustModPathInUse`) was
being included in the group list, and thus that all identifiers were
being highlighted as `Include`, which is definitely not what we wanted.

src/etc/vim/syntax/rust.vim

@@ -3,7 +3,7 @@
 " Maintainer:   Patrick Walton <pcwalton@mozilla.com>
 " Maintainer:   Ben Blum <bblum@cs.cmu.edu>
 " Maintainer:   Chris Morgan <me@chrismorgan.info>
-" Last Change:  July 06, 2014
+" Last Change:  July 18, 2014
 
 if version < 600
   syntax clear
@@ -38,7 +38,7 @@ syn keyword   rustKeyword     for in if impl let
 syn keyword   rustKeyword     loop once proc pub
 syn keyword   rustKeyword     return super
 syn keyword   rustKeyword     unsafe virtual while
-syn keyword   rustKeyword     use nextgroup=rustModPath skipwhite skipempty
+syn keyword   rustKeyword     use nextgroup=rustModPath,rustModPathInUse skipwhite skipempty
 " FIXME: Scoped impl's name is also fallen in this category
 syn keyword   rustKeyword     mod trait struct enum type nextgroup=rustIdentifier skipwhite skipempty
 syn keyword   rustStorage     mut ref static const
@@ -142,8 +142,9 @@ syn keyword   rustBoolean     true false
 " If foo::bar changes to foo.bar, change this ("::" to "\.").
 " If foo::bar changes to Foo::bar, change this (first "\w" to "\u").
 syn match     rustModPath     "\w\(\w\)*::[^<]"he=e-3,me=e-3
-syn match     rustModPath     "\w\(\w\)*" contained " only for 'use path;'
+syn match     rustModPathInUse "\w\(\w\)*" contained " only for 'use path;'
 syn match     rustModPathSep  "::"
+" rustModPathInUse is split out from rustModPath so that :syn-include can get the group list right.
 
 syn match     rustFuncCall    "\w\(\w\)*("he=e-1,me=e-1
 syn match     rustFuncCall    "\w\(\w\)*::<"he=e-3,me=e-3 " foo::<T>();
@@ -262,6 +263,7 @@ hi def link rustReservedKeyword Error
 hi def link rustConditional   Conditional
 hi def link rustIdentifier    Identifier
 hi def link rustCapsIdent     rustIdentifier
+hi def link rustModPathInUse  rustModPath
 hi def link rustModPath       Include
 hi def link rustModPathSep    Delimiter
 hi def link rustFunction      Function