Re: [PATCH v4] Teach git var about GIT_EDITOR

Jonathan Nieder [off-list ref] writes:

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.

Ok, so the idea is...

 * 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.

The above all look sensible, but IIRC, the true "vi" fell back on "ex"
mode on dumb terminals and was usable as a line editor, so we should be
able to run it even on dumb terminals.  I do not know about vi-clones
though.