rebase: verify commit parameter

If the user specifies a base commit to switch to, check if it actually
references a commit right away to avoid getting confused later on when
it turns out to be an invalid object.

Reported-by: LeSeulArtichaut <[email protected]>
Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

builtin/rebase.c

@@ -1906,7 +1906,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			die_if_checked_out(buf.buf, 1);
 			options.head_name = xstrdup(buf.buf);
 		/* If not is it a valid ref (branch or commit)? */
-		} else if (!get_oid(branch_name, &options.orig_head))
+		} else if (!get_oid(branch_name, &options.orig_head) &&
+			   lookup_commit_reference(the_repository,
+						   &options.orig_head))
 			options.head_name = NULL;
 		else
 			die(_("fatal: no such branch/commit '%s'"),