mergetools/meld: improve compatibiilty with Meld on macOS X

The macOS X fork of Meld[1] requires a "=" in the "--output" argument, as it uses a wrapper[2] script that munges the "--output" argument before calling into the common "meld" script. The macOS X wrapper script[2] accepts "--output=<filename>" only, despite the fact that the underlying meld code accepts both "--output <filename" and "--output=<filename>"[3]. All versions of meld which accept "--output" accept it in the "--output=<filename>" form, so use "--output=<file>" for maximum compatibility. [1] https://github.com/yousseb/meld [2] https://github.com/yousseb/meld/blob/master/osx/Meld [3] https://github.com/yousseb/meld/issues/42 Reported-by: Matthew Groth <mgroth49@gmail.com> Helped-by: Samuel Lijin <sxlijin@gmail.com> Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>