time-util: fix build with gcc8 -Werror=format-truncation=

time-util: fix build with gcc8 -Werror=format-truncation=

* it fails with gcc8 when -O1 or -Os is used (and -ftree-vrp which is added by -O2 and higher isn't used)

../git/src/basic/time-util.c: In function 'format_timespan':
../git/src/basic/time-util.c:508:46: error: '%0*llu' directive output between 1 and 2147483647 bytes may cause result to exceed 'INT_MAX' [-Werror=format-truncation=]
                                              "%s"USEC_FMT".%0*"PRI_USEC"%s",
                                              ^~~~
../git/src/basic/time-util.c:508:60: note: format string is defined here
                                              "%s"USEC_FMT".%0*"PRI_USEC"%s",
../git/src/basic/time-util.c:508:46: note: directive argument in the range [0, 18446744073709551614]
                                              "%s"USEC_FMT".%0*"PRI_USEC"%s",
                                              ^~~~
../git/src/basic/time-util.c:507:37: note: 'snprintf' output 4 or more bytes (assuming 2147483651) into a destination of size 4294967295
                                 k = snprintf(p, l,
                                     ^~~~~~~~~~~~~~
                                              "%s"USEC_FMT".%0*"PRI_USEC"%s",
                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                              p > buf ? " " : "",
                                              ~~~~~~~~~~~~~~~~~~~
                                              a,
                                              ~~
                                              j,
                                              ~~
                                              b,
                                              ~~
                                              table[i].suffix);
                                              ~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

[zj: change 'char' to 'signed char']