Investigate -Wformat-signedness

[Denton-L]

According to Peff, compiling with -Wformat-signedness might reveal some places where there output might be incorrectly formatted. (https://public-inbox.org/git/20190411234920.GA27914@sigill.intra.peff.net/)

Compile Git with -Wformat-signedness and fix some (or all) of these warnings? Maybe consider turning on the warning in config.mak.dev after these are fixed?