completions/git: Add some options for init.defaultBranch

[ci skip]

share/completions/git.fish

@@ -2138,6 +2138,8 @@ function __fish_git_complete_key_values
     switch $config_key
         case diff.algorithm
             printf "%s\n" myers patience histogram minimal
+        case init.defaultBranch
+            printf "%s\n" master main trunk dev next
         case '*'
             __fish_complete_path
     end