Support building deb packages from .dsc, fetching git branches and commits, & appending git commit revisions to git-based packages

[mmstick]

Closes #33

• dsc files may now be specified as a possible source location variant
• The git source location variant now supports specifying a branch and commit
• Enable automatic changelog updates of git sources, to append the commit to the changelog

Example

[DEBUG] debrep: running "git" "-C" "/home/mmstick/Sources/repo-curated-free/build/cosmic/piserver" "rev-parse" "master"
[DEBUG] debrep: running "dch" "-D" "cosmic" "-l" "~40a78a" "-c" "/home/mmstick/Sources/repo-curated-free/build/cosmic/piserver/debian/changelog" "automatic build of commit 40a78a"
[INFO] debrep: building piserver at changelog version 0.3~40a78a1

building ion-shell at changelog version 1.0.0-alpha0-cosmic2~0bff9e1

[[source]]
name = "ion-shell"
version = "1.0.0-alpha0-cosmic2"

    [source.location]
    dsc = "https://launchpad.net/~mmstick76/+archive/ubuntu/${name}/+sourcefiles/${name}/${version}/${name}_${version}.dsc"

[[source]]
name = "ion-shell"
build_on = "changelog"

    [source.location]
    git = "https://gitlab.redox-os.org/redox-os/ion/"
    branch = "master"
    commit = "0bff9e12"