completion: bash: fix gitk alias regression

Long time ago when the _git_complete helper was introduced, _gitk was
replaced with __gitk_main, and a placeholder for backwards compatibility
pointing to __git_wrap_main_gitk was left in place.

When "__git_complete gitk __gitk_main" was called, that created the
__git_wrap__gitk_main helper, which is just basically "__git_func_wrap
__gitk_main" plus `complete` options.

Unfortunately the commit b0a4b2d257 (completion: add support for
backwards compatibility, 2012-05-19) missed a previous instance of a
call to _gitk in _git_gitk

So, basically we had __git_wrap__git_main -> __git_func_wrap __git_main ->
__git_complete_command gitk -> _git_gitk -> _gitk ->
__git_wrap__gitk_main -> __git_func_wrap __gitk_main -> __gitk_main.

There was never any need to call __git_func_wrap twice. Since _git_gitk
is always called inside the wrapper, it can call __gitk_main directly.

And then, in commit 441ecdab37 (completion: bash: remove old compat
wrappers, 2020-10-27) _gitk was removed, which triggers the following
error:

  _git_gitk:9: command not found: _gitk

Let's call the correct function: __gitk_main.

Cc: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1806,7 +1806,7 @@ _git_fsck ()
 
 _git_gitk ()
 {
-	_gitk
+	__gitk_main
 }
 
 # Lists matching symbol names from a tag (as in ctags) file.