[DRE-maint] Bug#818261: git usage in gemspec

[Matthias Klose]

Package: src:ruby-github-markdown
Version: 1.3.3+dfsg-1
Severity: serious
Tags: sid stretch
Justification: Breaks rdeps when binary-rebuilt

When rebuiling ruby-github-markdown in unstable, the .gemspec is missing in
the built binaries. This causes rdeps to fail to resolve their
dependencies.

It's unclear how this package ever managed to get a .gemspec into
the binary.

see buildlog at
https://launchpadlibrarian.net/228973061/buildlog_ubuntu-xenial-amd64.ruby-github-markup_1.3.3+dfsg-1_BUILDING.txt.gz