oid: support sha256 in tostr_s

Our thread-local sha string buffer should be sized to support SHA256
strings.

src/libgit2/threadstate.h

@@ -13,7 +13,7 @@ typedef struct {
 	git_error *last_error;
 	git_error error_t;
 	git_str error_buf;
-	char oid_fmt[GIT_OID_SHA1_HEXSIZE+1];
+	char oid_fmt[GIT_OID_MAX_HEXSIZE+1];
 } git_threadstate;
 
 extern int git_threadstate_global_init(void);