This answer improves on a couple above (please vote them up), assuming that in addition to creating the one commit (no-parents no-history), you also want to retain all of the commit-data of that commit:
- Author (name and email)
- Authored date
- Commiter (name and email)
- Committed date
- Commmit log message
Of course the commit-SHA of the new/single commit will change, because it represents a new (non-)history, becoming a parentless/root-commit.
This can be done by reading git log and setting some variables for git commit-tree. Assuming that you want to create a single commit from master in a new branch one-commit, retaining the commit-data above:
git checkout -b one-commit master ## create new branch to reset
git reset --hard \
$(eval "$(git log master -n1 --format='\
COMMIT_MESSAGE="%B" \
GIT_AUTHOR_NAME="%an" \
GIT_AUTHOR_EMAIL="%ae" \
GIT_AUTHOR_DATE="%ad" \
GIT_COMMITTER_NAME="%cn" \
GIT_COMMITTER_EMAIL="%ce" \
GIT_COMMITTER_DATE="%cd"')" 'git commit-tree master^{tree} <<COMMITMESSAGE
$COMMIT_MESSAGE
COMMITMESSAGE
')