Fix Nix build header: F* rev empty

use fstar.exe --version instead of
git rev-parse HEAD in $FSTAR_HOME directory

lib/Driver.ml

@@ -44,6 +44,9 @@ module P = Process
 (* Note: don't open [Process], otherwise any reference to [Output] will be
  * understood as a cyclic dependency on our own [Output] module. *)
 
+(* TODO: raise this further? *)
+let git_rev_length = 10
+
 (** These three variables filled in by [detect_fstar] *)
 let fstar = ref ""
 let fstar_home = ref ""
@@ -181,7 +184,7 @@ let detect_karamel () =
     if try Sys.is_directory (krml_home ^^ ".git") with Sys_error _ -> false then begin
       let cwd = Sys.getcwd () in
       Sys.chdir krml_home;
-      krml_rev := String.sub (read_one_line "git" [| "rev-parse"; "HEAD" |]) 0 8;
+      krml_rev := String.sub (read_one_line "git" [| "rev-parse"; "HEAD" |]) 0 git_rev_length;
       Sys.chdir cwd
     end;
 
@@ -250,16 +253,12 @@ let detect_fstar () =
       KPrint.bprintf "%sfstar lib converted to windows path:%s %s\n" Ansi.underline Ansi.reset !fstar_lib
   end;
 
-  if try Sys.is_directory (!fstar_home ^^ ".git") with Sys_error _ -> false then begin
-    let cwd = Sys.getcwd () in
-    Sys.chdir !fstar_home;
-    let branch = read_one_line "git" [| "rev-parse"; "--abbrev-ref"; "HEAD" |] in
-    fstar_rev := String.sub (read_one_line "git" [| "rev-parse"; "HEAD" |]) 0 8;
-    let color = if branch = "master" then Ansi.green else Ansi.orange in
-    if not !Options.silent then
-      KPrint.bprintf "fstar is on %sbranch %s%s\n" color branch Ansi.reset;
-    Sys.chdir cwd
-  end;
+  (* As fstar.exe path is known, use fstar.exe --version flag *)
+  let fstar_version_output = Process.read_stdout !fstar [| "--version" |] in
+  (* fstar.exe --version currently yields a few lines,
+     fifth line is of the form commit=<git revision> *)
+  let fstar_rev_full = List.nth fstar_version_output 4 in
+  fstar_rev := String.sub fstar_rev_full (String.length "commit=") git_rev_length;
 
   let fstar_includes = List.map expand_prefixes !Options.includes in
   fstar_options := [