Fix command completion

The problem was that if the start of the input had a space (the "space"
test in command_completion.cpp), the parser wouldn't return it back
(wouldn't roll back the iterator).

Change-Id: Ifdf750c05bac7b92b36a5b2ca2b68624ae8dd8f1
diff --git a/src/grammars.hpp b/src/grammars.hpp
index 39b993d..6d7f11b 100644
--- a/src/grammars.hpp
+++ b/src/grammars.hpp
@@ -346,6 +346,9 @@
     x3::eps;
 
 auto const command_def =
+#if BOOST_VERSION <= 107700
+    x3::eps >>
+#endif
     createCommandSuggestions >> x3::expect[cd | copy | create | delete_rule | set | commit | get | ls | discard | describe | help | move | dump | prepare | exec | cancel | switch_rule];
 
 #if __clang__