support for disable .gitignore generation

i did for myself env hack:
https://github.com/pld-linux/cvs-fast-export/commit/ea069dbee87aef7fe8749d026b73f310223841cb

i.e if DISABLE_GITIGNORE=1 is present in env the .gitignore generation is skipped.

ideally this should be boolean commandline flag and perhaps option to specify your own .gitignore template