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] yousseb/meld#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>

mergetools/meld

@@ -10,7 +10,7 @@ merge_cmd () {
 
 	if test "$meld_has_output_option" = true
 	then
-		"$merge_tool_path" --output "$MERGED" \
+		"$merge_tool_path" --output="$MERGED" \
 			"$LOCAL" "$BASE" "$REMOTE"
 	else
 		"$merge_tool_path" "$LOCAL" "$MERGED" "$REMOTE"