fix(blame): show same commit twice or more

[antoinemadec]

Fixes #1121

issue

1. show_commit() always creates a new buffer and set its name according to the SHA1
2. that buffer is never wiped
3. when calling show_commit() twice on the same commit, we end up trying to create a 2nd buffer with the same name as a preexisting one

Error executing vim.schedule lua callback: ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:95: The async coroutine failed: ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/blame.lua:194: Failed to rename buffer
stack traceback:
        [C]: in function 'nvim_buf_set_name'
        ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/blame.lua:194: in function <...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/blame.lua:188>
stack traceback:
        [C]: in function 'error'
        ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:95: in function 'cb'
        ...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:145: in function <...cal/share/nvim/lazy/gitsigns.nvim/lua/gitsigns/async.lua:144>

fix

1. if preexisting buffer with right name: reuse it; else: create a new one
2. wipe the buffer when closing window

how it was tested

• Gitsigns blame, press s multiple tiems on the same commit
• check buffer is not present/listed after its window(s) are closed (same behavior as vim-fugitive)

[przepompownia]

@antoinemadec thanks for fixing before I found time for reporting this bug.