Fix Nix build header: F* rev empty

use fstar.exe --version instead of
git rev-parse HEAD in $FSTAR_HOME;
bump git revision length from 8 to 12

lib/Driver.ml

@@ -181,7 +181,7 @@ 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 12;
       Sys.chdir cwd
     end;
 
@@ -250,16 +250,13 @@ 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,
+     one is of the form commit=<git revision> *)
+  fstar_rev := List.hd (KList.filter_map
+    (fun s -> if KString.starts_with s "commit=" then Some (String.sub s (String.length "commit=") 12) else None)
+    fstar_version_output);
 
   let fstar_includes = List.map expand_prefixes !Options.includes in
   fstar_options := [