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>

Author: Ramsay Jones

builtin/revert.c

@@ -110,7 +110,7 @@ static void verify_opt_compatible(const char *me, const char *base_opt, ...)
 
 static void verify_opt_mutually_compatible(const char *me, ...)
 {
-	const char *opt1, *opt2;
+	const char *opt1, *opt2 = NULL;
 	va_list ap;
 
 	va_start(ap, me);

tree-walk.c

@@ -344,7 +344,7 @@ int traverse_trees(int n, struct tree_desc *t, struct traverse_info *info)
 		unsigned long mask, dirmask;
 		const char *first = NULL;
 		int first_len = 0;
-		struct name_entry *e;
+		struct name_entry *e = NULL;
 		int len;
 
 		for (i = 0; i < n; i++) {