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Dangerfile: Fix segfault when computing change size with renamed files

Patrick Steinhardt requested to merge pks-dangerfile-fix-changes-size-with-renames into master

We're quite regularly seeing failures in our Dangerfile CI job that looks like the following:

[!] Invalid `Dangerfile` file: \e[31m
[!] Invalid `Dangerfile` file: undefined method `[]' for nil:NilClass\e[0m
 #  from danger/rules/changes_size/Dangerfile:8
 #  -------------------------------------------
 #
 >    git.info_for_file(file)[:insertions]
 #  end
 #  -------------------------------------------
\e[0m
 #  from danger/rules/changes_size/Dangerfile:8
 #  -------------------------------------------
 #      %r{\\Adoc/.*(\\.(md|png|gif|jpg))\\z} => :docs,
 >      %r{\\A(CONTRIBUTING|LICENSE|README|REVIEWING|STYLE)(\\.md)?\\z} => :docs,
 #
 #  -------------------------------------------

This failure happens when we're trying to find out how many lines of code have changed while filtering out some specific files like generated Protobuf files.

Most notably, this failure seems to happen whenever the commit under test contains renamed files. What probably happens is that it cannot find the old name of the renamed file anymore, and consequentially it will fail with a nil pointer exception.

Fix this bug by explicitly checking for the case where we haven't found a specific file in Git.

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