commit message rewriting breaks with signed commits

[cgwalters]

See projectatomic/container-storage-setup#179 ➡️ projectatomic/container-storage-setup@f7a3746

[cgwalters]

From a quick google search:

http://stackoverflow.com/questions/31552774/unexpected-behavior-with-git-filter-branch-and-signed-commits
https://www.spinics.net/lists/git/msg256557.html

[Manishearth]

I was going to say that we never call git-filter-branch directly, and rebase handles this, so it's probably a bug in a git python lib we use, but it turns out we do use filter-branch 😄

[cgwalters]

Our previous Homu instance was on Fedora 23 (yes really) with an old git, but our new one (f25) has a version of git which has the fix for this.

So yes, anyone who hits this - upgrade your git.