Resolve a bug that Infer does not analyze procedures when the analyzer is run on '/'.

Summary:Resolve a bug that Infer does not analyze procedures when the analyzer is run on '/'.
This bug is reported by btakeya on facebook/infer#283.
Closes https://github.com/facebook/infer/pull/284

Reviewed By: cristianoc

Differential Revision: D2960328

Pulled By: jvillard

fb-gh-sync-id: 55e8b0c
shipit-source-id: 55e8b0c

infer/src/backend/DB.ml

@@ -51,9 +51,9 @@ let abs_source_file_from_path fname =
 
 (** convert a project root directory and a full path to a rooted source file *)
 let rel_source_file_from_abs_path root fname =
-  if Utils.string_is_prefix root fname then
-    let relative_fname = filename_to_relative root fname in
-    Relative relative_fname
+  let relative_complemented_fname = filename_to_relative root fname in
+  if Utils.string_is_prefix root fname && Filename.is_relative relative_complemented_fname then
+    Relative relative_complemented_fname
   else
     (* The project root is not a prefix of the file name *)
     abs_source_file_from_path fname