Merge branch 'jc/mergetool-tool-help' into maint

* jc/mergetool-tool-help:
  mergetool: support --tool-help option like difftool does

Documentation/git-mergetool.txt

@@ -27,9 +27,9 @@ OPTIONS
 -t <tool>::
 --tool=<tool>::
 	Use the merge resolution program specified by <tool>.
-	Valid merge tools are:
-	araxis, bc3, diffuse, ecmerge, emerge, gvimdiff, kdiff3,
-	meld, opendiff, p4merge, tkdiff, tortoisemerge, vimdiff and xxdiff.
+	Valid values include emerge, gvimdiff, kdiff3,
+	meld, vimdiff, and tortoisemerge. Run `git mergetool --tool-help`
+	for the list of valid <tool> settings.
 +
 If a merge resolution program is not specified, 'git mergetool'
 will use the configuration variable `merge.tool`.  If the

git-mergetool--lib.sh

@@ -111,7 +111,7 @@ run_merge_tool () {
 	return $status
 }
 
-guess_merge_tool () {
+list_merge_tool_candidates () {
 	if merge_mode
 	then
 		tools="tortoisemerge"
@@ -136,6 +136,10 @@ guess_merge_tool () {
 		tools="$tools emerge vimdiff"
 		;;
 	esac
+}
+
+guess_merge_tool () {
+	list_merge_tool_candidates
 	echo >&2 "merge tool candidates: $tools"
 
 	# Loop over each candidate and stop when a valid merge tool is found.

git-mergetool.sh

@@ -8,7 +8,7 @@
 # at the discretion of Junio C Hamano.
 #
 
-USAGE='[--tool=tool] [-y|--no-prompt|--prompt] [file to merge] ...'
+USAGE='[--tool=tool] [--tool-help] [-y|--no-prompt|--prompt] [file to merge] ...'
 SUBDIRECTORY_OK=Yes
 OPTIONS_SPEC=
 TOOL_MODE=merge
@@ -284,11 +284,51 @@ merge_file () {
     return 0
 }
 
+show_tool_help () {
+	TOOL_MODE=merge
+	list_merge_tool_candidates
+	unavailable= available= LF='
+'
+	for i in $tools
+	do
+		merge_tool_path=$(translate_merge_tool_path "$i")
+		if type "$merge_tool_path" >/dev/null 2>&1
+		then
+			available="$available$i$LF"
+		else
+			unavailable="$unavailable$i$LF"
+		fi
+	done
+	if test -n "$available"
+	then
+		echo "'git mergetool --tool=<tool>' may be set to one of the following:"
+		echo "$available" | sort | sed -e 's/^/	/'
+	else
+		echo "No suitable tool for 'git mergetool --tool=<tool>' found."
+	fi
+	if test -n "$unavailable"
+	then
+		echo
+		echo 'The following tools are valid, but not currently available:'
+		echo "$unavailable" | sort | sed -e 's/^/	/'
+	fi
+	if test -n "$unavailable$available"
+	then
+		echo
+		echo "Some of the tools listed above only work in a windowed"
+		echo "environment. If run in a terminal-only session, they will fail."
+	fi
+	exit 0
+}
+
 prompt=$(git config --bool mergetool.prompt || echo true)
 
 while test $# != 0
 do
     case "$1" in
+	--tool-help)
+		show_tool_help
+		;;
 	-t|--tool*)
 	    case "$#,$1" in
 		*,*=*)