WordPress .gitignore - this is my preferred gitignore file when working with WordPress. It ignores almost all files by default.

.gitignore

# -----------------------------------------------------------------
# .gitignore for WordPress @salcode
# ver 20150227
#
# From the root of your project run
# curl -O https://gist.githubusercontent.com/salcode/b515f520d3f8207ecd04/raw/.gitignore
# to download this file
#
# By default all files are ignored.  You'll need to whitelist
# any mu-plugins, plugins, or themes you want to include in the repo.
#
# To ignore uncommitted changes in a file that is already tracked, use 
# git update-index --assume-unchanged
#
# To stop tracking a file that is currently tracked, use 
# git rm --cached
#
# Change Log:
# 20150302 Added composer.json as a whitelisted file
# 20150227 Created as fork of https://gist.github.com/salcode/9940509,
#   this version ignores all files by default
# -----------------------------------------------------------------

# ignore everything in the root except the "wp-content" directory.
/*
!wp-content/

# ignore everything in the "wp-content" directory, except:
# mu-plugins, plugins, and themes directories
wp-content/*
!wp-content/mu-plugins/
!wp-content/plugins/
!wp-content/themes/

# ignore all mu-plugins, plugins, and themes
# unless explicitly whitelisted at the end of this file
wp-content/mu-plugins/*
wp-content/plugins/*
wp-content/themes/*

# ignore all files starting with . or ~
.*
~*

# ignore node dependency directories (used by grunt)
node_modules/

# ignore OS generated files
ehthumbs.db
Thumbs.db

# ignore Editor files
*.sublime-project
*.sublime-workspace
*.komodoproject

# ignore log files and databases
*.log
*.sql
*.sqlite

# ignore compiled files
*.com
*.class
*.dll
*.exe
*.o
*.so

# ignore packaged files
*.7z
*.dmg
*.gz
*.iso
*.jar
*.rar
*.tar
*.zip

# -------------------------
# BEGIN Whitelisted Files
# -------------------------

# track these files, if they exist
!.gitignore
!.editorconfig
!README.md
!CHANGELOG.md
!composer.json

# track these mu-plugins, plugins, and themes
# add your own entries here
!wp-content/mu-plugins/example-mu-plugin/
!wp-content/plugins/example-plugin/
!wp-content/themes/example-theme/