get_main_worktree(): allow it to be called in the Git directory

When called in the Git directory of a non-bare repository, this function
would not return the directory of the main worktree, but of the Git
directory instead.

The reason: when the Git directory is the current working directory, the
absolute path of the common directory will be reported with a trailing
`/.git/.`, which the code of `get_main_worktree()` does not handle
correctly.

Let's fix this.

Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

worktree.c

@@ -51,6 +51,7 @@ static struct worktree *get_main_worktree(void)
 	struct strbuf worktree_path = STRBUF_INIT;
 
 	strbuf_add_absolute_path(&worktree_path, get_git_common_dir());
+	strbuf_strip_suffix(&worktree_path, "/.");
 	if (!strbuf_strip_suffix(&worktree_path, "/.git"))
 		strbuf_strip_suffix(&worktree_path, "/.");