How do you make `git grep` output look like `ack` output?

I've recently found git grep and come to like its speed and de facto searching of only the files in the repo. But coming from ack (ack-grep in Ubuntu), one thing left to be desired is the output formatting, which is unfortunately much more like grep than ack. Go figure.

ack:

1. Prints the matching filename on the first line by itself.
2. Color highlights the matching filename a bold green.
3. Prints the line number, and only the line number, with each matching line.
4. Color highlights the line number a bold yellow.
5. Color highlights each matching string a background yellow.
6. Prints a blank line between matches from different files.

On the other hand, git grep:

• Prints the filename on every matching line.
• Prints no line number.
• Prints no blank line between matches from different files.
• Color highlights only the matching text, a bold red.

Is there any set of git grep options, or combo with other tools, that can make git grep output look like ack output?

You've already answered part of your own question (--break inserts a blank line between files, --heading prints the file name separately, and -n or --line-number gives you line numbers on each line).

The rest is just color options, which are set in git config via the color.grep.<slot> entries. See the documentation for full details, but note that based on what you asked for, I think this does the trick:

[alias]
    ack = -c color.grep.linenumber=\"bold yellow\" \
          -c color.grep.filename=\"bold green\" \
          -c color.grep.match=\"reverse yellow\" \
          grep --break --heading --line-number

(this is expressed as you'd see it in git config --global --edit since the quoting is messy).

Or, to set it up in one command:

git config --global alias.ack '-c color.grep.linenumber="bold yellow"
    -c color.grep.filename="bold green"
    -c color.grep.match="reverse yellow"
    grep --break --heading --line-number'

Add or subtract -c options to change whatever colors you like, and/or set them to your preferred defaults by setting color.grep.<name> = color instead of using the git ack alias.