cmd/govim: fix GOVIMQuickfixDiagnostics command (#662)

Per #657, after 2975a8f `:GOVIMQuickfixDiagnostics` is broken as long as
QuickfixAutoDiagnostics is disabled. This was an unintentional break. We
managed to break this command because the command was not actually
tested.

Fix both of these points by fixing the bug itself and also adding a test
to prevent further breakage.

Fixes #657

cmd/govim/quickfix.go

@@ -21,7 +21,7 @@ func (v *vimstate) quickfixDiagnostics(flags govim.CommandFlags, args ...string)
 }
 
 func (v *vimstate) updateQuickfix(diags []types.Diagnostic, force bool) error {
-	if v.config.QuickfixAutoDiagnostics == nil || !*v.config.QuickfixAutoDiagnostics {
+	if !force && (v.config.QuickfixAutoDiagnostics == nil || !*v.config.QuickfixAutoDiagnostics) {
 		return nil
 	}
 	v.diagnosticsChangedLock.Lock()

cmd/govim/testdata/scenario_default/quickfix_config.txt

@@ -59,6 +59,13 @@ cmp errors empty
 vim -indent expr 'sign_getplaced(\"main.go\", {\"group\": \"*\"})'
 cmp stdout signs.golden3
 
+# Calling :GOVIMQuickfixDiagnostics should force-populate the quickfix window
+vim ex 'GOVIMQuickfixDiagnostics'
+vim ex 'copen'
+vim ex 'w errors'
+vim ex 'cclose'
+cmp errors errors.golden3
+
 # Disabled pending resolution to https://github.com/golang/go/issues/34103
 # errlogmatch -start -count=0 'LogMessage callback: &protocol\.LogMessageParams\{Type:(1|2), Message:".*'
 
@@ -239,6 +246,15 @@ main.go|11 col 19| missing return
     ]
   }
 ]
+-- errors.golden3 --
+main.go|6 col 36| undeclared name: i
+main.go|6 col 39| undeclared name: v
+main.go|7 col 35| undeclared name: i
+main.go|7 col 38| undeclared name: v
+main.go|8 col 35| undeclared name: i
+main.go|8 col 38| undeclared name: v
+main.go|11 col 19| missing return
+main.go|12 col 19| missing return
 -- nosigns.golden --
 [
   {