Activate flyspell-mode to git-commit-mode

flyspell-mode was not being activated with magit-log-edit-mode-hook,
now the hook is for git-commit-mode, which activates flyspell-mode when
editing commit message with magit.
Activating flyspell-mode by default to check spelling could not always
catch all misspellings, considering using ispell on region to check all
possible misspellings.