Fix off-by-one problems in git_signature__parse
Fix off-by-one problems in git_signature__parse

Etc/GMT-14 aka UTC+14:00 is a thing....
https://en.wikipedia.org/wiki/UTC%2B14:00

Also allow offsets on the last minute (59).

Addresses: https://bugs.debian.org/841532
Fixes: #3970

stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2,stewid/libgit2