GitRepository diff: Force histogram diff algorithm (Closes #1120136)

Gioele Barabuccirequested to merge
gioele/git-buildpackage:git-diff-algo-histogram into master

The default algorithm refers to the myers algorithm in Git 2.x, but will be changed to refer to the histogram algorithm in Git 3.x.

Force the use of the (better) histogram algorithm to avoid future churn due to developers using gbp-pq with different major versions of Git.

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