Added support for Mac OS X and other variants of Unix that do not use GNU version of readlink and sed.

- The BSD version of readlink does not support the -f option.

- The BSD version of sed does do not support specifying “-” for the filename when receiving input from standard input.

- Improved performance and UNIX portability of pre-commit-uncrustify by removing the use of sed and grep when determining that committed file's extension matches the ones to be sent to uncrustify.