Performance optimization: `yield` with an implicit `block` is faster than `block.call`.
See http://youtu.be/fGFM_UrSp70?t=10m35s and the following benchmark:
```ruby
require 'benchmark/ips'
def fast
yield
end
def slow(&block)
block.call
end
Benchmark.ips do |x|
x.report('fast') { fast{} }
x.report('slow') { slow{} }
end
# => fast 154095 i/100ms
# => slow 71454 i/100ms
# =>
# => fast 7511067.8 (±5.0%) i/s - 37445085 in 4.999660s
# => slow 1227576.9 (±6.8%) i/s - 6145044 in 5.028356s
```
This Thor-action isolates the logic whether to (over-)write migration and
what is shown to the user. It's modelled after Thor's CreateFile-action.
This solves the issue that removing a non-existing migration, tried to
remove the template-path (#13588).
Related issues: #12674
