shada/context: fully remove jumplist duplicates #10898

- Always load files when cleaning up jumplist.
  - For Shada: avoids writing duplicate entries, which happens when you read
    from a shada file with duplicate entries (merging the jumplist while
    writing sometimes produces duplicate entries, bug?) and then write right
    away (i.e.: without any `:jumps`, `getjumplist()`, or any jump movement,
    that is: nothing that calls `cleanup_jumplist` with `loadfiles == true`).
  - For Context: avoids non-idempotent behavior for the same reason (i.e.:
    first call to `shada_encode_jumps` does not remove duplicate entries).

- Do not set pcmark when dumping jumplist for Context.
  - Retrieving current Context shouldn't add an entry to the jumplist
    (which will be removed by a subsequent `cleanup_jumplist` anyway, i.e.:
    tail entry matching current position), just act like `getjumplist` for
    instance.

src/nvim/shada.c

@@ -2737,6 +2737,8 @@ static ShaDaWriteResult shada_write(ShaDaWriteDef *const sd_writer,
   }
 
   // Initialize jump list
+  setpcmark();
+  cleanup_jumplist(curwin, false);
   wms->jumps_size = shada_init_jumps(wms->jumps, &removable_bufs);
 
   // Initialize global marks
@@ -4086,11 +4088,13 @@ static bool shada_removable(const char *name)
 static inline size_t shada_init_jumps(
     PossiblyFreedShadaEntry *jumps, khash_t(bufset) *const removable_bufs)
 {
-  // Initialize jump list
+  if (!curwin->w_jumplistlen) {
+    return 0;
+  }
+
   size_t jumps_size = 0;
   const void *jump_iter = NULL;
-  setpcmark();
-  cleanup_jumplist(curwin, false);
+
   do {
     xfmark_T fm;
     jump_iter = mark_jumplist_iter(jump_iter, curwin, &fm);
@@ -4164,6 +4168,7 @@ void shada_encode_jumps(msgpack_sbuffer *const sbuf)
   khash_t(bufset) removable_bufs = KHASH_EMPTY_TABLE(bufset);
   find_removable_bufs(&removable_bufs);
   PossiblyFreedShadaEntry jumps[JUMPLISTSIZE];
+  cleanup_jumplist(curwin, true);
   size_t jumps_size = shada_init_jumps(jumps, &removable_bufs);
   msgpack_packer packer;
   msgpack_packer_init(&packer, sbuf, msgpack_sbuffer_write);