launch_editor(): allow spaces in the filename

launch_editor(): allow spaces in the filename

The construct

	sh -c "$0 \"$@\"" <editor> <file>

does not pick up quotes in <editor>, so you cannot give path to the
editor that has a shell IFS whitespace in it, and also give it initial
set of parameters and flags.  Replace $0 with <editor> to fix this issue.

This fixes

	git config core.editor '"c:/Program Files/What/Ever.exe"'

In other words, you can specify an editor with spaces in its path using a
config containing something like this:

	[core]
		editor = \"c:/Program Files/Darn/Spaces.exe\"

NOTE: we cannot just replace the $0 with \"$0\", because we still want
this to work:

	[core]
		editor = emacs -nw

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>