merge-tree: fix argument type of the --merge-base option

In 5f43cf5 (merge-tree: accept 3 trees as arguments, 2024-01-28), I
taught `git merge-tree` to perform three-way merges on trees. This
commit even changed the manual page to state that the `--merge-base`
option takes a tree-ish rather than requiring a commit.

But I forgot to adjust the in-program help text. This patch fixes that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/merge-tree.c

@@ -563,7 +563,7 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 			   PARSE_OPT_NONEG),
 		OPT_STRING(0, "merge-base",
 			   &merge_base,
-			   N_("commit"),
+			   N_("tree-ish"),
 			   N_("specify a merge-base for the merge")),
 		OPT_STRVEC('X', "strategy-option", &xopts, N_("option=value"),
 			N_("option for selected merge strategy")),