git: Set .gitignore to ignore downloaded files, but not their sha1 files.

Git won't track the files that are downloaded into big/, so there's no reason
for it to tell people about them. Their sha1 files, however, are tracked and
changes in their status shouldn't be ignored.

Change-Id: I2e0157f7742eac905d04f1032b792f549bc4f099
1 file changed