vim-patch:8.1.0550: expression evaluation may repeat an error message

Problem:    Expression evaluation may repeat an error message. (Jason
            Franklin)
Solution:   Increment did_emsg and check for the value when giving an error
            for the echo command.
76a6345433

src/nvim/eval.c

@@ -19538,6 +19538,7 @@ void ex_echo(exarg_T *eap)
   typval_T rettv;
   bool needclr = true;
   bool atstart = true;
+  const int did_emsg_before = did_emsg;
 
   if (eap->skip)
     ++emsg_skip;
@@ -19552,7 +19553,7 @@ void ex_echo(exarg_T *eap)
         // Report the invalid expression unless the expression evaluation
         // has been cancelled due to an aborting error, an interrupt, or an
         // exception.
-        if (!aborting()) {
+        if (!aborting() && did_emsg == did_emsg_before) {
           EMSG2(_(e_invexpr2), p);
         }
         need_clr_eos = false;

src/nvim/message.c

@@ -505,7 +505,7 @@ int emsg(const char_u *s_)
      */
     if (cause_errthrow((char_u *)s, severe, &ignore) == true) {
       if (!ignore) {
-        did_emsg = true;
+        did_emsg++;
       }
       return true;
     }
@@ -554,7 +554,7 @@ int emsg(const char_u *s_)
     } else {
       flush_buffers(FLUSH_MINIMAL);  // flush internal buffers
     }
-    did_emsg = true;          // flag for DoOneCmd()
+    did_emsg++;               // flag for DoOneCmd()
   }
 
   emsg_on_display = true;     // remember there is an error message

test/functional/legacy/108_backtrace_debug_commands_spec.lua

@@ -121,7 +121,6 @@ describe('108', function()
       Error detected while processing function Foo[2]..Bar[2]..Bazz:
       line    3:
       E121: Undefined variable: var3
-      E15: Invalid expression: var3
       here var3 is defined with "another var":
       another var
 
@@ -129,7 +128,6 @@ describe('108', function()
       Error detected while processing function Foo[2]..Bar:
       line    3:
       E121: Undefined variable: var2
-      E15: Invalid expression: var2
       here var2 is defined with 10:
       10