Simplify Git resource's get_revision method

[pocke]

This pull request simplifies get_revision method of Itamae::Resource::Git.
It will improve performance.

This pull request contains two changes.

First, it replaces git rev-list with git rev-parse.
git rev-parse is faster, and enough. rev-list prints all revisions in the specified branch, but rev-parse only prints the head revision.

It is a really simple benchmark.

require 'benchmark'
Benchmark.bm do |x|
  x.report{100.times{`git rev-parse @`}}
  x.report{100.times{`git rev-list @`}}
end

# In itamae repository
#
#        user     system      total        real
#    0.012472   0.020599   0.245997 (  0.235425)
#    0.124308   0.398403   2.054095 (  1.553693)

Second, it reduces git command executions when git fetch is unnecessary.

---

I guess this pull request only has a small impact on performance because git resource always try fetching new commits from the remote repository by default.

[pocke]

I unexpectedly removed stdout method call, so I've fixed it in cb116c2. 🙈
I'll squash the commits if you need it.

[unasuke]

💨

[unasuke]

@pocke Thank you! I'll release a new version after merge your pull requests.