Provide completion for git subcommands (#1044)

This is to provide completion for when user types `git [Tab]`.
There is already completion for `git help [Tab]` but I don't know why
author don't add it to `git` alone.

Fixes #1043

custom-completions/git/git-completions.nu

@@ -798,3 +798,7 @@ export extern "git grep" [
   --quiet(-q)                           # Do not output matched lines; instead, exit with status 0 when there is a match and with non-zero status when there isn’t.
   ...pathspecs: string                  # Target pathspecs to limit the scope of the search.
 ]
+
+export extern "git" [
+  command?: string@"nu-complete git subcommands"       # subcommand to show help for
+]