git grep -E doesn't accept \b word boundaries?

[Kevin Ushey <kevinushey@gmail.com>]

Hello,

I'm seeing the following, which I believe is unexpected. I have a file
with contents:

$ cat hello.txt
WholeWord
Whole Word
Whole

I can use `git grep` to search with word boundaries; e.g.

$ git grep --untracked '\bWhole\b'
hello.txt:Whole Word
hello.txt:Whole

However, if I add `-E` to use extended regular expressions, the same
invocation finds no search results.

$ git grep --untracked -E '\bWhole\b'

This does seem to work as expected with the '-w' flag, e.g.

$ git grep --untracked -E -w 'Whole'
hello.txt:Whole Word
hello.txt:Whole

as well as with POSIX word boundaries, e.g.

$ git grep --untracked -E '[[:<:]]Whole[[:>:]]'
hello.txt:Whole Word
hello.txt:Whole

Is this a bug, or am I misunderstanding some behavior in `git grep`?
For posterity:

$ git grep --untracked -G '\bWhole\b'
hello.txt:Whole Word
hello.txt:Whole

$ git grep --untracked -E '\bWhole\b'

$ git grep --untracked -P '\bWhole\b'
hello.txt:Whole Word
hello.txt:Whole

For what it's worth, I don't see this issue with an older version of
`git` on an Ubuntu 22.04 VM:

root@96722b73f316:~/test# git --version
git version 2.34.1
root@96722b73f316:~/test# git grep --untracked -E '\bWhole\b'
hello.txt:Whole Word
hello.txt:Whole

Thanks,
Kevin

------

[System Info]
git version:
git version 2.40.1
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