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To exclude everything in a directory, but some sub-directories, do the following:
wp-content/*
!wp-content/plugins/
!wp-content/themes/
Source: https://gist.github.com/444295
Just add ! before an exclusion rule.
According to the gitignore man page:
Patterns have the following format:
...
- An optional prefix ! which negates the pattern; any matching file excluded by a previous pattern will become included again. If a negated pattern matches, this will override lower precedence patterns sources.