mergetools/meld: improve compatibiilty with Meld on macOS X

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>