libgimp: fix issue #6050 Phantom comments on pictures.

Since version 0.27.3 exiv2 has changed how it returns
comments for Exif.Photo.UserComment. We now get
the comment including the charset=Ascii value.

Let's remove anything that's not part of the actual
comment. To not complicate things we will  only
handle charset=Ascii for now since I've never seen
any other charset used.

libgimp/gimpimagemetadata.c

@@ -37,9 +37,10 @@ typedef struct
   gint  type;
 } XmpStructs;
 
+static gchar *     gimp_image_metadata_interpret_comment    (gchar *comment);
 
-static void        gimp_image_metadata_rotate        (GimpImage         *image,
+static void        gimp_image_metadata_rotate               (GimpImage         *image,
-                                                      GExiv2Orientation  orientation);
+                                                             GExiv2Orientation  orientation);
 
 /*  public functions  */
 
@@ -80,6 +81,37 @@ gimp_image_metadata_load_prepare (GimpImage    *image,
   return metadata;
 }
 
+static gchar *
+gimp_image_metadata_interpret_comment (gchar *comment)
+{
+  /* Exiv2 can return unwanted text at the start of a comment
+   * taken from Exif.Photo.UserComment since 0.27.3.
+   * Let's remove that part and return NULL if there
+   * is nothing else left as comment. */
+
+  if (comment && g_str_has_prefix (comment, "charset=Ascii "))
+    {
+      gchar *real_comment;
+
+      /* Skip "charset=Ascii " (length 14) to find the real comment */
+      real_comment = comment + 14;
+      if (real_comment[0] == '\0' ||
+          ! g_strcmp0 (real_comment, "binary comment"))
+        {
+          g_free (comment);
+          return NULL;
+        }
+      else
+        {
+          real_comment = g_strdup (real_comment);
+          g_free (comment);
+          return real_comment;
+        }
+    }
+
+  return comment;
+}
+
 /**
  * gimp_image_metadata_load_finish:
  * @image:       The image
@@ -109,6 +141,8 @@ gimp_image_metadata_load_finish (GimpImage             *image,
 
       comment = gexiv2_metadata_get_tag_interpreted_string (GEXIV2_METADATA (metadata),
                                                             "Exif.Photo.UserComment");
+      comment = gimp_image_metadata_interpret_comment (comment);
+
       if (! comment)
         comment = gexiv2_metadata_get_tag_interpreted_string (GEXIV2_METADATA (metadata),
                                                               "Exif.Image.ImageDescription");