Fix flaky construction of genrule $SRCDIR

When the project root is initialized to "." the absolute paths for
genrule() srcs can be constructed as /Users/self/src/./path.  This
later confuses genrule when it looks for the prefix to strip when
creating the $SRCDIR, the canonical path of a source never contains
/./ as a component.
diff --git a/src/com/facebook/buck/cli/Main.java b/src/com/facebook/buck/cli/Main.java
index d0aa030..1db964f 100644
--- a/src/com/facebook/buck/cli/Main.java
+++ b/src/com/facebook/buck/cli/Main.java
@@ -349,7 +349,7 @@
 
   public static void main(String[] args) {
     Main main = new Main(System.out, System.err);
-    File projectRoot = new File(".");
+    File projectRoot = new File("").getAbsoluteFile();
     int exitCode = FAIL_EXIT_CODE;
     try {
       exitCode = main.tryRunMainWithExitCode(projectRoot, args);