mergetool: Make ECMerge use the settings as specified by the user in the GUI

mergetool: Make ECMerge use the settings as specified by the user in the GUI

When run from the command line, ECMerge does not automatically use the same
settings for a merge / diff that it would use when starting the GUI and loading
files manually. In the first case the built-in factory defaults would be used,
while in the second case the settings the user has specified in the GUI would
be used, which can be misleading. Specifying the "--default" command line
option changes this behavior so that always the user specfified GUI settings
are used.

Signed-off-by: Sebastian Schuberth <sschuberth@visageimaging.com>
Tested-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>