bug#23075: 24.5; vc-git-command should use coding-system-for-read

[Nikolay Kudryavtsev]

Hello.

Ok, here are more detailed steps on how to repeat this this bug.
1. Clone https://github.com/sg2002/vc-git-bugs
2. Open 1251.txt in windows1251 encoding. Do some changes in it.
3. vc-diff. This would give you diff with broken encoding.
4. Evaluate vc-git-command from fix.el in the repo.
5. vc-diff again, it would give you the correct encoding as long as the 
file itself opened correctly.

Yes, there is some code that specifies encoding in vc already. See the 
first large comment in vc-diff-internal.

This issue is not platform specific, unlike 23076.

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Best Regards,
Nikolay Kudryavtsev