Replace $() bashism with backticks

[phdru]

No description provided.

[joelparkerhenderson]

Thanks for the idea. This project aims to use the git coding guidelines.

https://github.com/git/git/blob/6f92e5ff3cdc813de8ef5327fd4bad492fb7d6c9/Documentation/CodingGuidelines#L72-L74

"We prefer $( ... ) for command substitution; unlike ``, it properly nests. It should have been the way Bourne spelled it from day one, but unfortunately isn't."

Here's a link that helps show the $(...) on more shells than just bash, and also in the POSIX shell.

http:https://stackoverflow.com/questions/4708549/whats-the-difference-between-command-and-command-in-shell-programming

I'll have this pull request open for a week or so if people want to comment on it.