Various fix for finder's prompt

[ofseed]

This PR includes three commits, explained here.

1. Change the default prompt_prefix value

The default value of the prompt_prefix is an indicator " > ": https://github.com/NeogitOrg/neogit/blob/0d0879b0045fb213c328126969a3317c0963d34a/lua/neogit/lib/finder.lua#L152

But whenever the Finder was created, it added an indicator as a postfix also: https://github.com/NeogitOrg/neogit/blob/0d0879b0045fb213c328126969a3317c0963d34a/lua/neogit/lib/finder.lua#L182

So the default value of prompt_prefix for users will be " > > ", I change the default value to a more meaningful value in this commit.

2. Remove redundant prompt_prefix postfix

The behavior of adding a postfix for every prompt_prefix has problems too. Because neogit do it whenever the Finder is created: https://github.com/NeogitOrg/neogit/blob/0d0879b0045fb213c328126969a3317c0963d34a/lua/neogit/lib/finder.lua#L182 And when the finder is not telescope.nvim, it will add the postfix again: https://github.com/NeogitOrg/neogit/blob/0d0879b0045fb213c328126969a3317c0963d34a/lua/neogit/lib/finder.lua#L224 https://github.com/NeogitOrg/neogit/blob/0d0879b0045fb213c328126969a3317c0963d34a/lua/neogit/lib/finder.lua#L233

So this behavior is redundant in this situation. There are two ways to fix this indeed,

1. Do not add the postfix whenever the Finder is created;
2. Do not add the postfix when the Finder is not telescope.nvim also.

I selected the second way to complete the third commit.

3. Modify postfix for finders

For telescope.nvim, maybe the prefix space is needed, but for fzf-lua, it is redundant, so I removed it, and for vim.ui.select, even though there's no standard to specify what the prompt postfix is, most plugins select a colon. Most importantly, the popular plugin dressing.nvim which is dedicated to customizing vim.ui.* will trim if the end is a colon in some situations to be more beautiful.

[CKolkey]

Nice.