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.

test/functional/api/vim_spec.lua

@@ -922,11 +922,9 @@ describe('API', function()
         },
 
         ['jumps'] = eval(([[
-        filter(map(add(
-        getjumplist()[0], { 'bufnr': bufnr('%'), 'lnum': getcurpos()[1] }),
-        'filter(
-        { "f": expand("#".v:val.bufnr.":p"), "l": v:val.lnum },
-        { k, v -> k != "l" || v != 1 })'), '!empty(v:val.f)')
+        filter(map(getjumplist()[0], 'filter(
+          { "f": expand("#".v:val.bufnr.":p"), "l": v:val.lnum },
+          { k, v -> k != "l" || v != 1 })'), '!empty(v:val.f)')
         ]]):gsub('\n', '')),
 
         ['buflist'] = eval([[