vcs: Ensure vcs-git protects user from git stderr.

This change ensures that stderr is not printed when an exact match, as
expected, is not found.  The stderr from git was confusing to users.

Author: tee3

src/tools/vcs-git.jam

@@ -51,7 +51,14 @@ rule generate-version-string
     errors.user-error "vcs-git: $(directory) is not a Git repository." ;
   }
 
-  v = [ SHELL "(cd $(directory) && git describe --tags --exact-match --dirty)" ] ;
+  if [ os.name ] = NT
+  {
+    v = [ SHELL "(cd $(directory) && git describe --tags --exact-match --dirty) 2> NUL" ] ;
+  }
+  else
+  {
+    v = [ SHELL "(cd $(directory) && git describe --tags --exact-match --dirty) 2> /dev/null" ] ;
+  }
   v = [ SPLIT_BY_CHARACTERS $(v) : "\n" ] ;
 
   if $(v) != ""