Provide a build time default-editor setting

Provide a build time default-editor setting

Provide a DEFAULT_EDITOR knob to allow setting the fallback
editor to use instead of vi (when VISUAL, EDITOR, and GIT_EDITOR
are unset).  The value can be set at build time according to a
system’s policy.  For example, on Debian systems, the default
editor should be the 'editor' command.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>