Fix commit-msg hook to allow editing

The old git-commit.sh script allowed the commit-msg hook to not only
prevent a commit from proceding, but also to edit the commit message
on the fly and allow it to proceed. So here we teach builtin-commit
to do the same.

This is based on Wincent's patch, but redone with a clarified logic.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -787,16 +787,17 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		char index[PATH_MAX];
 		const char *env[2] = { index, NULL };
 		snprintf(index, sizeof(index), "GIT_INDEX_FILE=%s", index_file);
-		launch_editor(git_path(commit_editmsg), &sb, env);
-	} else if (strbuf_read_file(&sb, git_path(commit_editmsg), 0) < 0) {
-		rollback_index_files();
-		die("could not read commit message");
+		launch_editor(git_path(commit_editmsg), NULL, env);
 	}
 	if (!no_verify &&
 	    run_hook(index_file, "commit-msg", git_path(commit_editmsg))) {
 		rollback_index_files();
 		exit(1);
 	}
+	if (strbuf_read_file(&sb, git_path(commit_editmsg), 0) < 0) {
+		rollback_index_files();
+		die("could not read commit message");
+	}
 
 	/* Truncate the message just before the diff, if any. */
 	p = strstr(sb.buf, "\ndiff --git a/");

builtin-tag.c

@@ -53,6 +53,8 @@ void launch_editor(const char *path, struct strbuf *buffer, const char *const *e
 			die("There was a problem with the editor %s.", editor);
 	}
 
+	if (!buffer)
+		return;
 	if (strbuf_read_file(buffer, path, 0) < 0)
 		die("could not read message file '%s': %s",
 		    path, strerror(errno));