snapshots: use the upload date instead of the commit date as the title

[dscho]

The snapshots at https://gitforwindows.org/git-snapshots/ have a header that describes the git-for-windows/git commit on which each snapshot is based.

Sometimes I create multiple snapshots for the same commit, though, e.g. to ensure that we a have testable version after a MSYS2 runtime upgrade that does not also move to another git-for-windows/git commit. In these cases, the date that is shown is still reflecting when the git-for-windows/git commit was created. It would be better to use the upload date instead, or maybe the one reflecting when the snapshot was built (for which existing items could be adjusted by using the newest timestamp stored in the corresponding PortableGit artifact, for example).