[sledge] Do not always output list of bitcode inputs

Summary:
Outputting the list of bitcode inputs when no output flag is ok for
`sledge buck bitcode` but does not make sense when it is composed as
part of other commands. So only output to stdout if `-` is given as
the output file name.

Reviewed By: kren1

Differential Revision: D16059782

fbshipit-source-id: abac9c36f

sledge/sledge-help.txt

@@ -49,7 +49,8 @@ Analyze code in a buck target. This is a convenience wrapper for the sequence `s
   [-function-summaries]     use function summaries (in development)
   [-lib-fuzzer]             add a harness for lib fuzzer binaries
   [-output-llair <file>]    write generated LLAIR to <file>
-  [-output-modules <file>]  write list of bitcode files to <file>
+  [-output-modules <file>]  write list of bitcode files to <file>, or to
+                            standard output if <file> is `-`
   [-skip-throw]             do not explore past throwing an exception
   [-trace <spec>]           enable debug tracing
   [-help]                   print this help text and exit
@@ -66,7 +67,8 @@ Build a buck target and report the included bitcode files.
 
 === flags ===
 
-  [-output-modules <file>]  write list of bitcode files to <file>
+  [-output-modules <file>]  write list of bitcode files to <file>, or to
+                            standard output if <file> is `-`
   [-trace <spec>]           enable debug tracing
   [-help]                   print this help text and exit
                             (alias: -?)

sledge/src/sledge_buck.ml

@@ -176,15 +176,18 @@ let main ~(command : unit Command.basic_command) ~analyze =
     let%map_open target = target_flag
     and output =
       flag "output-modules" (optional string)
-        ~doc:"<file> write list of bitcode files to <file>"
+        ~doc:
+          "<file> write list of bitcode files to <file>, or to standard \
+           output if <file> is `-`"
     in
     let bitcode_files = bitcode_files_of ~target in
     ( match output with
-    | Some file -> Out_channel.write_lines file bitcode_files
-    | None ->
+    | Some "-" ->
         Format.printf "%a"
           (List.pp " " Format.pp_print_string)
-          bitcode_files ) ;
+          bitcode_files
+    | Some file -> Out_channel.write_lines file bitcode_files
+    | None -> () ) ;
     bitcode_files
   in
   let bitcode_cmd =