[rubygems/rubygems] Avoid allocating empty hashes in Index

Since the hashes have a default proc that returns a (new) empty hash, we
can avoid allocating those empty hashes when we are only doing lookups.

Test from running `bundle update --bundler` against a rails app I have
lying around:

```
==> memprof.after.txt <==
Total allocated: 9.71 MB (68282 objects)
Total retained:  4.87 MB (33791 objects)

==> memprof.before.txt <==
Total allocated: 10.83 MB (100596 objects)
Total retained:  5.02 MB (34721 objects)
```

https://github.com/rubygems/rubygems/commit/8f7c9cb23e

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/bundler/index.rb b/lib/bundler/index.rb
index 6a20302322..1b2d3b5333 100644
--- a/lib/bundler/index.rb
+++ b/lib/bundler/index.rb
@@ -179,7 +179,8 @@ module Bundler
     end
 
     def specs_by_name(name)
-      @specs[name].values
+      return EMPTY_SEARCH unless specs = @specs.fetch(name, nil)
+      specs.values
     end
 
     EMPTY_SEARCH = [].freeze
@@ -190,11 +191,13 @@ module Bundler
     end
 
     def find_by_spec(spec)
-      @specs[spec.name][spec.full_name]
+      return unless specs = @specs.fetch(spec.name, nil)
+      specs[spec.full_name]
     end
 
     def exist?(spec)
-      @specs[spec.name].key?(spec.full_name)
+      return unless specs = @specs.fetch(spec.name, nil)
+      specs.key?(spec.full_name)
     end
   end
 end