rename git-define-git-commit-self to git-commit-define-header-inserter

Likewise rename `git-define-git-commit-self'
to `git-commit-define-self-header-inserter'.

The new names aren't all that nice, mostly way to long.  But at least
they actual provide a hint at what they are supposed to do and begin
with the correct name prefix.
allow find-function to find definitions of header inserters
with-editor-emacsclient-executable: add to correct Custom group