Make dependencies target less noisy when run repeatedly

BUG=none
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/309933005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21639 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

Makefile

@@ -468,7 +468,7 @@ dependencies:
 	svn checkout --force \
 	    https://src.chromium.org/chrome/trunk/deps/third_party/icu46 \
 	    third_party/icu --revision 258359
-	( cd buildtools || \
+	( test -d buildtools || \
 	  git clone https://chromium.googlesource.com/chromium/buildtools.git; \
 	  cd buildtools; \
 	  git fetch origin; \