Git LFS patterns with [[:space:]] are not parsed correctly.

Reproduction steps:

1. Create a git repo with lfs and add a big file with a whitespace in it (for instance, "big file.bmp", which size is several MBs)
2. Run:

   git lfs track "big file.bmp"

1. Open .gitattributes file. Observe that whitespace was automatically replaced by [[:space:]]

big[[:space:]]image.bmp filter=lfs diff=lfs merge=lfs -text

1. Open Source Tree and try to stage this file. Observe the following warning: "The following files are 2 MB or larger and are not currently tracked in Git LFS."