Defining timestamp_t as intmax_t instead of uintmax_t

timestamp_t is a data type defined in git-compat-util.h as uintmax_t since 28f4aee3fb (use uintmax_t for timestamps, 2017-04-26). It was introduced with dddbad728c (timestamp_t: a new data type for timestamps, 2017-04-26) to replace use of both time_t and unsigned long for time-related uses.

On Linux with glibc[1] and on Windows with MSVC[2], time_t is always a signed type. I think this is also the case on MacOS and FreeBSD. Thus, any conversion or comparison between the two types is one between a signed and an unsigned type. This also means git cannot deal with time before 1970, though I can't think of a scenario where it will have to. However, it does it to sign compare warnings from the compiler.

I propose swapping uintmax_t with intmax_t to fix this problem. I do not think that this will cause any problem unless any code sets the most significant bit of the timestamp for some special purpose. However, the fix will take some work, considering the number of references to timestamp_t. So I want to get everyone's opinions on whether this should be done before starting work on it.

[1] https://www.gnu.org/software/libc/manual/html_node/Time-Types.html [2] https://learn.microsoft.com/en-us/cpp/c-runtime-library/standard-types?view=msvc-170