Subject of the issue

Unicode characters in filenames breaks diffing

Your environment

• git version 2.20.1
• git (1.5.0)
• ruby 2.6.4p104 (2019-08-28 revision 67798) [x86_64-linux]

Steps to reproduce

$ mkdir -p /tmp/test && cd /tmp/test
$ git init
$ printf '\xE2\x98\xA0' # print a skull and crossbones symbol
$ touch 'some_file'
$ git add .
$ git commit -m"init"
$ touch 'my_other_file_\xE2\x98\xA0'
$ git add .
$ git commit -m"commit"

git = Git.init '/tmp/test'
git.diff('@^').each { |x| ... }

Expected behaviour

The diff to be produced as usual

Actual behaviour

NoMethodError: undefined method `[]' for nil:NilClass

The first line of a diff is usually something like git --diff a/somefile b/somefile, but when the filename contains a unicode character, the filenames are escaped, (e.g. diff --git "a/my_other_file_\\xE2\\x98\\xA0" "b/my_other_file_\\xE2\\x98\\xA0") and this regex fails to detect the line.