`git-commit-save-message': only save commit msg if it contains actual text

Avoid filling up `log-edit-comment-ring' with empty commit messages
by checking whether the commit message contains any actual text
(i.e. word chars that aren't part of a comment) before saving it.

Signed-off-by: Pieter Praet <pieter@praet.org>
`git-commit-save-message': only save commit msg if it's unique

Avoid filling up `log-edit-comment-ring' with duplicate commit messages
(and potentially clobbering useful history in the process) when cycling
through it using `git-commit-{prev,next}-message'.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #70 from praet/commit-save-msg

Commit message history fixes