git-commit should be explicitly --short=7

[Sergio Benjamim]

git-commit variable should be explicitly --short=7, since git 2.10/2.11 changes the behavior of --short, depending on the number of objects in the repository it can gives more characters.

ProblemType: Bug
DistroRelease: Ubuntu 16.10
Package: git-build-recipe 0.3.3
ProcVersionSignature: Ubuntu 4.8.0-22.24-generic 4.8.0
Uname: Linux 4.8.0-22-generic x86_64
ApportVersion: 2.20.3-0ubuntu8
Architecture: amd64
CasperVersion: 1.379
CurrentDesktop: XFCE
Date: Thu Jan 26 17:57:48 2017
LiveMediaBuild: Xubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.2)
PackageArchitecture: all
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: git-build-recipe
UpgradeStatus: No upgrade log present (probably fresh install)