parse-options: introduce precision handling for OPTION_UNSIGNED

This commit is the equivalent to the preceding commit, but instead of
introducing precision handling for `OPTION_INTEGER` we introduce it for
`OPTION_UNSIGNED`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse.h

@@ -2,6 +2,7 @@
 #define PARSE_H
 
 int git_parse_signed(const char *value, intmax_t *ret, intmax_t max);
+int git_parse_unsigned(const char *value, uintmax_t *ret, uintmax_t max);
 int git_parse_ssize_t(const char *, ssize_t *);
 int git_parse_ulong(const char *, unsigned long *);
 int git_parse_int(const char *value, int *ret);