mergetool: support --tool-help option like difftool does

[Junio C Hamano <gitster@xxxxxxxxx>]

This way we do not have to risk the list of tools go out of sync
between the implementation and the documentation.

Signed-off-by: Junio C Hamano <gitster@xxxxxxxxx>
---
Junio C Hamano <gitster@xxxxxxxxx> writes:

>> +--tool-help::
>> +	List the supported and available diff tools.
>
> This part is a good addition (but it already is mentioned in the
> description of --tool above, so it is more or less a "Meh").

I noticed that the main while loop has style violations in its
case/esac, but I left it as-is.  Reindenting it would be a low
hanging fruit janitor patch that would be a separate topic.

 git-mergetool--lib.sh |  6 +++++-
 git-mergetool.sh      | 42 +++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index ed630b2..f730253 100644
--- a/git-mergetool--lib.sh
+++ b/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.
diff --git a/git-mergetool.sh b/git-mergetool.sh
index a9f23f7..0db0c44 100755
--- a/git-mergetool.sh
+++ b/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
 		*,*=*)
--
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