nvim: init.vim: Remove twiggy & switch to merginal

Twiggy does not seem to work with fish shell. See the below
issue and considering that it has been open since more than
a year does not look likely to be solved.
https://github.com/sodapopcan/vim-twiggy/issues/7

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>

nvim/init.vim

@@ -30,7 +30,7 @@ Plug 'lervag/vimtex'
 Plug 'tpope/vim-fugitive'
 Plug 'junegunn/gv.vim'
 Plug 'rhysd/git-messenger.vim'
-Plug 'sodapopcan/vim-twiggy'
+Plug 'idanarye/vim-merginal'
 Plug 'christoomey/vim-conflicted'
 Plug 'rbong/vim-flog'
 " For tmux yank