Teach git var about GIT_EDITOR

Teach git var about GIT_EDITOR

Expose the command used by launch_editor() for scripts to use.
This should allow one to avoid searching for a proper editor
separately in each command.

git_editor(void) uses the logic to decide which editor to use
that used to live in launch_editor().  The function returns NULL
if there is no suitable editor; the caller is expected to issue
an error message when appropriate.

launch_editor() uses git_editor() and gives the error message the
same way as before when EDITOR is not set.

"git var GIT_EDITOR" gives the editor name, or an error message
when there is no appropriate one.

"git var -l" gives GIT_EDITOR=name only if there is an
appropriate editor.

Originally-submitted-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>