Fix some "variable might be used uninitialized" warnings

Fix some "variable might be used uninitialized" warnings

In particular, gcc complains as follows:

        CC tree-walk.o
    tree-walk.c: In function `traverse_trees':
    tree-walk.c:347: warning: 'e' might be used uninitialized in this \
        function

        CC builtin/revert.o
    builtin/revert.c: In function `verify_opt_mutually_compatible':
    builtin/revert.c:113: warning: 'opt2' might be used uninitialized in \
        this function

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>