mergetool--lib: simplify command expressions

Update variable assignments to always use $(command "$arg")
in their RHS instead of "$(command "$arg")" as the latter
is harder to read.  Make get_merge_tool_cmd() simpler by
avoiding "echo" and $(command) substitutions completely.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -32,17 +32,10 @@ check_unchanged () {
 	fi
 }
 
-valid_tool_config () {
-	if test -n "$(get_merge_tool_cmd "$1")"
-	then
-		return 0
-	else
-		return 1
-	fi
-}
-
 valid_tool () {
-	setup_tool "$1" || valid_tool_config "$1"
+	setup_tool "$1" && return 0
+	cmd=$(get_merge_tool_cmd "$1")
+	test -n "$cmd"
 }
 
 setup_tool () {
@@ -96,14 +89,13 @@ setup_tool () {
 }
 
 get_merge_tool_cmd () {
-	# Prints the custom command for a merge tool
 	merge_tool="$1"
 	if diff_mode
 	then
-		echo "$(git config difftool.$merge_tool.cmd ||
-			git config mergetool.$merge_tool.cmd)"
+		git config "difftool.$merge_tool.cmd" ||
+		git config "mergetool.$merge_tool.cmd"
 	else
-		echo "$(git config mergetool.$merge_tool.cmd)"
+		git config "mergetool.$merge_tool.cmd"
 	fi
 }
 
@@ -114,7 +106,7 @@ run_merge_tool () {
 	GIT_PREFIX=${GIT_PREFIX:-.}
 	export GIT_PREFIX
 
-	merge_tool_path="$(get_merge_tool_path "$1")" || exit
+	merge_tool_path=$(get_merge_tool_path "$1") || exit
 	base_present="$2"
 	status=0
 
@@ -145,7 +137,7 @@ run_merge_tool () {
 
 # Run a either a configured or built-in diff tool
 run_diff_cmd () {
-	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
+	merge_tool_cmd=$(get_merge_tool_cmd "$1")
 	if test -n "$merge_tool_cmd"
 	then
 		( eval $merge_tool_cmd )
@@ -158,11 +150,11 @@ run_diff_cmd () {
 
 # Run a either a configured or built-in merge tool
 run_merge_cmd () {
-	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
+	merge_tool_cmd=$(get_merge_tool_cmd "$1")
 	if test -n "$merge_tool_cmd"
 	then
-		trust_exit_code="$(git config --bool \
-			mergetool."$1".trustExitCode || echo false)"
+		trust_exit_code=$(git config --bool \
+			"mergetool.$1.trustExitCode" || echo false)
 		if test "$trust_exit_code" = "false"
 		then
 			touch "$BACKUP"
@@ -253,7 +245,7 @@ guess_merge_tool () {
 	# Loop over each candidate and stop when a valid merge tool is found.
 	for i in $tools
 	do
-		merge_tool_path="$(translate_merge_tool_path "$i")"
+		merge_tool_path=$(translate_merge_tool_path "$i")
 		if type "$merge_tool_path" >/dev/null 2>&1
 		then
 			echo "$i"
@@ -300,7 +292,7 @@ get_merge_tool_path () {
 	fi
 	if test -z "$merge_tool_path"
 	then
-		merge_tool_path="$(translate_merge_tool_path "$merge_tool")"
+		merge_tool_path=$(translate_merge_tool_path "$merge_tool")
 	fi
 	if test -z "$(get_merge_tool_cmd "$merge_tool")" &&
 		! type "$merge_tool_path" >/dev/null 2>&1
@@ -314,11 +306,11 @@ get_merge_tool_path () {
 
 get_merge_tool () {
 	# Check if a merge tool has been configured
-	merge_tool="$(get_configured_merge_tool)"
+	merge_tool=$(get_configured_merge_tool)
 	# Try to guess an appropriate merge tool if no tool has been set.
 	if test -z "$merge_tool"
 	then
-		merge_tool="$(guess_merge_tool)" || exit
+		merge_tool=$(guess_merge_tool) || exit
 	fi
 	echo "$merge_tool"
 }