Add petertriho/cmp-git plugin

Notionally this is supposed to provide GitHub issue/pull request
completions but so far I've only got mentions working.

lua/plugins/completions.lua

@@ -10,7 +10,7 @@ return {
     'hrsh7th/cmp-nvim-lsp', -- Source for built-in language server client
     'hrsh7th/cmp-buffer',   -- Source for buffer words
     'hrsh7th/cmp-path',     -- Source for filesystem paths
-    'hrsh7th/cmp-cmdline',  -- Source for command-line
+    'petertriho/cmp-git',   -- Source for Git/GitHub/GitLab
     'hrsh7th/nvim-cmp',     -- Completion engine combines and use the above
 
     -- LSP UI plugins
@@ -93,6 +93,7 @@ return {
       },
     })
 
+    require("cmp_git").setup({})
     local cmp = require('cmp')
     cmp.setup({
       snippet = {
@@ -118,7 +119,7 @@ return {
         { name = 'luasnip' },
         { name = 'buffer' },
         { name = 'path' },
-        -- { name = 'cmdline' },
+        { name = 'git' },
       },
 
       window = {