Gracefully handle gem loading errors

The relaxing of the `childprocess` gem dependency constraint in 89dc99a
resulted in us seeing the following error running `overcommit --run` in
the local Overcommit repository:

    .../lib/bundler/runtime.rb:317:in `check_for_activated_spec!':
    You have already activated childprocess 0.6.3, but your Gemfile
    requires childprocess 0.7.0. Prepending `bundle exec` to your
    command may solve this. (Gem::LoadError)

Since we didn't prepend `bundle exec` to the command, the version of
`overcommit` that is executed is whichever is the latest version
installed in the local set of installed gems. In the example above, I
had Overcommit 0.39.1 installed, which depends on childprocess ~> 0.6.3.
Thus `childprocess` 0.6.3 is loaded, and when we attempt to invoke
`Bundler.setup` we get the `Gem::LoadError` referenced above since the
local `Gemfile.lock` expects `childprocess` 0.7.0.

We can work around this by catching this specific error and re-execing
with `bundle exec` prepended to the command.

Author: sds

CHANGELOG.md

@@ -8,6 +8,9 @@
 * Use the `core.hooksPath` Git configuration option when installing hooks
 * Gracefully handle binary files in `LineEndings` pre-commit hook
 * Relax `childprocess` dependency to allow 0.x
+* Gracefully handle gem loading errors when invoking Overcommit in a repo where
+  the `gemfile` specified by the local `.overcommit.yml` references a gem
+  version incompatible with the already-loaded Overcommit
 
 ## 0.39.1
 

bin/overcommit

@@ -22,6 +22,12 @@ if gemfile = YAML.load_file('.overcommit.yml')['gemfile'] rescue nil
     puts "Problem loading '#{gemfile}': #{ex.message}"
     puts "Try running:\nbundle install --gemfile=#{gemfile}" if ex.is_a?(Bundler::GemNotFound)
     exit 78 # EX_CONFIG
+  rescue Gem::LoadError => ex
+    # Handle case where user is executing overcommit without `bundle exec` and
+    # whose local Gemfile has a gem requirement that does not match a gem
+    # requirement of the installed version of Overcommit.
+    raise unless ex.message =~ /already activated/i
+    exec('bundle', 'exec', $0, *ARGV)
   end
 end
 # rubocop:enable Style/RescueModifier