Update obsolete URLs in documentation

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>

HACKING.md

@@ -30,7 +30,7 @@ find an overview here:
 Next, clone the repository and all the associated libraries:
 
 ```
-$ git clone http://darcs.haskell.org/ghc.git/
+$ git clone http://git.haskell.org/ghc.git
 $ cd ghc
 $ ./sync-all --testsuite get
 ```

docs/users_guide/external_core.xml

@@ -158,7 +158,7 @@ I am unsure of the proper DocBook elements.
     <para>Formal static and dynamic semantics in the form of an executable
     typechecker and interpreter are available separately in the GHC
     source tree
-    <footnote><ulink url="http://darcs.haskell.org/ghc">http://darcs.haskell.org/ghc</ulink></footnote>
+    <footnote><ulink url="http://git.haskell.org/ghc.git/tree">http://git.haskell.org/ghc.git</ulink></footnote>
     under <code>utils/ext-core</code>.</para>
 
   </section>

docs/users_guide/glasgow_exts.xml

@@ -8966,7 +8966,7 @@ Because the preprocessor targets Haskell (rather than Core),
 patterns</emphasis>, written <literal>!<replaceable>pat</replaceable></literal>.
 Bang patterns are under consideration for Haskell Prime.
 The <ulink
-url="http://hackage.haskell.org/trac/haskell-prime/wiki/BangPatterns">Haskell
+url="http://ghc.haskell.org/trac/haskell-prime/wiki/BangPatterns">Haskell
 prime feature description</ulink> contains more discussion and examples
 than the material below.
 </para>