command: Fix 0.13upgrade to preserve more comments

Previously, any comments inside the required provider configuration for
a given provider would be wiped out upon rerunning the 0.13upgrade
command. This commit attempts to preserve those comments if the existing
entry is semantically equivalent to the entry we are about to write.

command/013_config_upgrade.go

@@ -404,7 +404,18 @@ command and dealing with them before running this command again.
 			} else {
 				attributesObject = cty.EmptyObjectVal
 			}
-			body.SetAttributeValue(localName, attributesObject)
+			// If this block already has an entry for this local name, we only
+			// want to replace it if it's semantically different
+			if existing := body.GetAttribute(localName); existing != nil {
+				bytes := existing.Expr().BuildTokens(nil).Bytes()
+				expr, _ := hclsyntax.ParseExpression(bytes, "", hcl.InitialPos)
+				value, _ := expr.Value(nil)
+				if !attributesObject.RawEquals(value) {
+					body.SetAttributeValue(localName, attributesObject)
+				}
+			} else {
+				body.SetAttributeValue(localName, attributesObject)
+			}
 
 			// If we don't have a source attribute, manually construct a commented
 			// block explaining what to do

command/testdata/013upgrade-preserves-comments/expected/main.tf

@@ -9,8 +9,15 @@ terraform {
       source  = "hashicorp/bar"
       version = "0.5.0"
     }
+    # An explicit requirement
+    baz = {
+      # Comment inside the block should stay
+      source = "foo/baz"
+    }
+    # Foo is required
     foo = {
-      source = "hashicorp/foo"
+      source  = "hashicorp/foo"
+      version = "1.0.0"
     }
   }
 }

command/testdata/013upgrade-preserves-comments/input/main.tf

@@ -6,5 +6,15 @@ terraform {
   required_providers {
     # Pin bar to this version
     bar = "0.5.0"
+    # An explicit requirement
+    baz = {
+      # Comment inside the block should stay
+      source = "foo/baz"
+    }
+    # Foo is required
+    foo = {
+      # This comment sadly won't make it
+      version = "1.0.0"
+    }
   }
 }