merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/git-mergetool--lib.sh

@@ -43,7 +43,14 @@ show_tool_names () {
 
 	shown_any=
 	( cd "$MERGE_TOOLS_DIR" && ls ) | {
-		while read toolname
+		while read scriptname
+		do
+			setup_tool "$scriptname" 2>/dev/null
+			variants="$variants$(list_tool_variants)\n"
+		done
+		variants="$(echo "$variants" | sort | uniq)"
+
+		for toolname in $variants
 		do
 			if setup_tool "$toolname" 2>/dev/null &&
 				(eval "$condition" "$toolname")
@@ -157,6 +164,10 @@ setup_tool () {
 		echo "$1"
 	}
 
+	list_tool_variants () {
+		echo "$tool"
+	}
+
 	# Most tools' exit codes cannot be trusted, so By default we ignore
 	# their exit code and check the merged file's modification time in
 	# check_unchanged() to determine whether or not the merge was
@@ -178,19 +189,26 @@ setup_tool () {
 		false
 	}
 
-
-	if ! test -f "$MERGE_TOOLS_DIR/$tool"
+	if test -f "$MERGE_TOOLS_DIR/$tool"
 	then
+		. "$MERGE_TOOLS_DIR/$tool"
+	elif test -f "$MERGE_TOOLS_DIR/${tool%[0-9]}"
+	then
+		. "$MERGE_TOOLS_DIR/${tool%[0-9]}"
+	else
 		setup_user_tool
 		return $?
 	fi
 
-	# Load the redefined functions
-	. "$MERGE_TOOLS_DIR/$tool"
 	# Now let the user override the default command for the tool.  If
 	# they have not done so then this will return 1 which we ignore.
 	setup_user_tool
 
+	if ! list_tool_variants | grep -q "^$tool$"
+	then
+		return 1
+	fi
+
 	if merge_mode && ! can_merge
 	then
 		echo "error: '$tool' can not be used to resolve merges" >&2
@@ -286,11 +304,14 @@ list_merge_tool_candidates () {
 		tools="$tools smerge"
 	fi
 	case "${VISUAL:-$EDITOR}" in
+	*nvim*)
+		tools="$tools nvimdiff vimdiff emerge"
+		;;
 	*vim*)
-		tools="$tools vimdiff emerge"
+		tools="$tools vimdiff nvimdiff emerge"
 		;;
 	*)
-		tools="$tools emerge vimdiff"
+		tools="$tools emerge vimdiff nvimdiff"
 		;;
 	esac
 }