Ensure any passed in string doesn't contains the `\0` character

[nulltoken]

Following discussion with @ethomson in #530 (comment) , some methods are now protected against the evil zero character (cf . 28d1820, for instance).

It would make sense to also protect every remaining string in the API that is eventually passed to libgit2.

[someoneigna]

Imposing a full check of each string passed to libgit2 seems unfair and becomes expensive. Shouldn't the consumer of libgit2sharp be the one responsible for that?

[dahlbyk]

Pending a profiler's input, "expensive" is relative when we're talking about walking mostly short strings (branch names, etc) compared with going to disk.

[someoneigna]

Something wrong went with my repo and commits got messed up. Things I learnt are:

• Dont have to reference issues on the commit, that goes on the pull request message.
• Issues section traces each commit even if you try to push force to rewrite the log. (Behaves differently than Pull requests)

Will open a proper pull request shortly, sorry for the mess.

[nulltoken]

Dont have to reference issues on the commit, that goes on the pull request message

Actually, having the reference to the issue that's being fixed in the commit message is a nice hint when trying to understand, months later, some why some changes happened.