mergetools/meld: make usage of `--output` configurable and more robust

Older versions of meld listed --output in `meld --help`.
Newer versions only mention `meld [OPTIONS...]`.
Improve the checks to catch these newer versions.

Add a `mergetool.meld.hasOutput` configuration to allow
overriding the heuristic.

Reported-by: Andrey Novoseltsev <novoselt@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1754,6 +1754,15 @@ mergetool.<tool>.trustExitCode::
 	if the file has been updated, otherwise the user is prompted to
 	indicate the success of the merge.
 
+mergetool.meld.hasOutput::
+	Older versions of `meld` do not support the `--output` option.
+	Git will attempt to detect whether `meld` supports `--output`
+	by inspecting the output of `meld --help`.  Configuring
+	`mergetool.meld.hasOutput` will make Git skip these checks and
+	use the configured value instead.  Setting `mergetool.meld.hasOutput`
+	to `true` tells Git to unconditionally use the `--output` option,
+	and `false` avoids using `--output`.
+
 mergetool.keepBackup::
 	After performing a merge, the original file with conflict markers
 	can be saved as a file with a `.orig` extension.  If this variable

mergetools/meld

@@ -18,13 +18,18 @@ merge_cmd () {
 	check_unchanged
 }
 
-# Check whether 'meld --output <file>' is supported
+# Check whether we should use 'meld --output <file>'
 check_meld_for_output_version () {
 	meld_path="$(git config mergetool.meld.path)"
 	meld_path="${meld_path:-meld}"
 
-	if "$meld_path" --help 2>&1 | grep -e --output >/dev/null
+	if meld_has_output_option=$(git config --bool mergetool.meld.hasOutput)
 	then
+		: use configured value
+	elif "$meld_path" --help 2>&1 |
+		grep -e '--output=' -e '\[OPTION\.\.\.\]' >/dev/null
+	then
+		: old ones mention --output and new ones just say OPTION...
 		meld_has_output_option=true
 	else
 		meld_has_output_option=false