setup ert tests
add ert tests for pseudo header position
test that comments aren't saved in message history
put all temp buffers for tests in git-commit-mode
non-comment AND non-empty line is start of message

In case there are blank lines between comment blocks, as happens after
merge+conflict, git-commit-find-pseudo-header-position should skip over
the blank lines too.
don't save comments in commit message

Only save and restore up to the pseudo-header position.
Merge branch 'pr/no-save-comment4'
Merge branch 'pr/test-msg-hist'