Speed up bundler/setup by using the raw Gemfile.lock information without extra processing whenever possible

[deivid-rodriguez]

What was the end-user or developer problem that led to this PR?

We should make bundler/setup as fast as possible, since it's the cost that all scripts using Bundler to select versions of dependencies pay.

What is your fix for the problem, implemented in this PR?

I noticed that even in the case where the Gemfile has no changes over the Gemfile.lock file, we're still doing some processing over the raw Gemfile.lock recorded resolution that did not seem necessary.

Further investigation revealed that this is only done for two edge cases:

• When dependencies are deleted from the Gemfile. In this case, we don't need to re-resolve, only to select the proper subset of the existing lockfile resolution.
• When the lockfile has redundant platform specific gems recorded. This can only happen to lockfiles generated with versions of Bundler prior to 1.16.1, due to this bug.

So my fix is to explicitly detect the above two situations and only do the extra processing there. Otherwise, use the raw lockfile.

This speeds bundling the Gemfile in https://github.com/technicalpickles/big-gemfile by about 5%, but should speed up all invocations of bundler/setup that don't need a re-resolve, the more dependencies, the more speed up.

➜  big-gemfile git:(main) ✗ hyperfine 'BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1' 'BUNDLER_VERSION=2.3.17 ruby -rbundler/setup -e1'  
Benchmark 1: BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1
  Time (mean ± σ):     146.3 ms ±   0.9 ms    [User: 121.6 ms, System: 23.3 ms]
  Range (min … max):   144.5 ms … 148.4 ms    19 runs
 
Benchmark 2: BUNDLER_VERSION=2.3.17 ruby -rbundler/setup -e1
  Time (mean ± σ):     153.2 ms ±   0.7 ms    [User: 128.9 ms, System: 22.9 ms]
  Range (min … max):   151.5 ms … 154.3 ms    19 runs
 
Summary
  'BUNDLER_VERSION=2.4.0.dev ruby -rbundler/setup -e1' ran
    1.05 ± 0.01 times faster than 'BUNDLER_VERSION=2.3.17 ruby -rbundler/setup -e1'

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