Dear maintainer,

Lintian tag new-package-uses-date-based-version-number recommends prefixing
upstream versions that are date-based with 0~. Unfortunately gbp buildpackage
then assumes the 0~ to be part of the upstream tag and fails to find it. For
example of Google's abseil library:

  gbp:error: upstream/0_20250512.1 is not a valid treeish

The correct upstream tag in this case would be upstream/20250512.1. The single
character substitution supported by the --git-upstream-tag option is
insufficient to strip the leading 0~.

It would be nice if the substitution format supported more complicated
operations or detected and stripped this case automatically, using presumably
the same detection logic as the Lintian warning.

Raul