gitignore: make `.ExternalData` exclusion more specific

When reformatting the entire tree, the `.ExternalData` directory ends up
running into issues. Instead of excluding the entire directory, exclude
the paths that are expected to be there.

.gitignore

@@ -2,5 +2,6 @@
 # script in the source directory.  There is no way to suppress their
 # creation short of copying the file somewhere.
 *.pyc
-.ExternalData*
+.ExternalData/config/
+.ExternalData/SHA512/
 .DS_Store