test diff::tests::format_email_simple fails, only in release mode

[joshtriplett]

While testing git2, I noticed that cargo test --release fails while cargo test succeeds. The test diff::tests::format_email_simple fails, because the generated email From line starts with:

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001

It has an all-zero hash instead of the actual commit hash. It's especially disturbing that this only happens in an optimized build.

This failure happens with either rustc 1.45.2 (d3fb005a3 2020-07-31) or rustc 1.47.0-nightly (8e21bd063 2020-08-14).

[joshtriplett]

@ehuss Thanks for the fix!