[PATCH v3] git-archive: Add new option "--output" to write archive to a file instead of stdout.

Patch attached as MIME to avoid conversion problems.

Patch highlights:
1. Formatted to 80 columns.
2. Change the language in the documentation to match the style.
3. Modified create_output_file.
3.1 Changed from creat(2) to open(2).
3.2 Changed the logic so we only dup the file if the fd is different from 1.
3.3 Didn't change the closing of the file, since dup2 will close the file for us.
Only close the file if there was an error.
4. Didn't change the tests, it never hurts to have many tests :-)

Regards