Git solutions

create_patch_from_commit.sh

git cherry-pick -n <commit> # get your patch, but don't commit (-n = --no-commit)
git reset                   # unstage the changes from the cherry-picked commit
git add -p                  # make all your choices (add the changes you do want)
git commit                  # make the commit!

modify_specific_commit.sh

# Use git rebase to set HEAD on commit bbc643cd
$ git rebase --interactive bbc643cd^

# In the default editor, modify 'pick' to 'edit' in the line whose commit you want to modify. 
# Make your changes and then stage them with
$ git add <filepattern>

# Now you can run to modify the commit
$ git commit --amend

# and after that to return back to the previous head commit.
$ git rebase --continue