Never use fast-forward merges

[lkysow]

When using --checkout-strategy merge, we were performing the merge
without the --no-ff flag. This meant that if the branch could be merged
via a fast-forward merge (without a merge commit), then later when we
ran git rev-parse HEAD^2 to determine if the repo was at the right
commit, it would fail.

Now we merge with --no-ff which ensures the commit tree will always look
the same and git rev-parse HEAD^2 will always succeed.

Fixes #584

[codecov]

Codecov Report

Merging #585 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #585      +/-   ##
==========================================
- Coverage   72.15%   72.13%   -0.03%     
==========================================
  Files          61       61              
  Lines        4629     4618      -11     
==========================================
- Hits         3340     3331       -9     
+ Misses       1044     1043       -1     
+ Partials      245      244       -1

Impacted Files	Coverage Δ
server/events/working_dir.go	74.69% <100%> (ø)	⬆️
cmd/server.go	78.97% <0%> (-0.06%)	⬇️
server/logging/simple_logger.go	0% <0%> (ø)	⬆️
server/events/runtime/run_step_runner.go	100% <0%> (ø)	⬆️

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