difftool--helper: Remove use of the GIT_MERGE_TOOL variable

An undocumented mis-feature in git-difftool is that it allows you
to specify a default difftool by setting GIT_MERGE_TOOL.
This behavior was never documented and was included as an
oversight back when git-difftool was maintained outside of git.

git-mergetool never honored GIT_MERGE_TOOL so neither should
git-difftool.

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

git-difftool--helper.sh

@@ -40,11 +40,10 @@ launch_merge_tool () {
 	run_merge_tool "$merge_tool"
 }
 
-# Allow GIT_DIFF_TOOL and GIT_MERGE_TOOL to provide default values
-test -n "$GIT_MERGE_TOOL" && merge_tool="$GIT_MERGE_TOOL"
-test -n "$GIT_DIFF_TOOL" && merge_tool="$GIT_DIFF_TOOL"
-
-if test -z "$merge_tool"; then
+# GIT_DIFF_TOOL indicates that --tool=... was specified
+if test -n "$GIT_DIFF_TOOL"; then
+	merge_tool="$GIT_DIFF_TOOL"
+else
 	merge_tool="$(get_merge_tool)" || exit
 fi
 

t/t7800-difftool.sh

@@ -32,7 +32,6 @@ restore_test_defaults()
 	# Restores the test defaults used by several tests
 	remove_config_vars
 	unset GIT_DIFF_TOOL
-	unset GIT_MERGE_TOOL
 	unset GIT_DIFFTOOL_PROMPT
 	unset GIT_DIFFTOOL_NO_PROMPT
 	git config diff.tool test-tool &&
@@ -107,15 +106,7 @@ test_expect_success 'GIT_DIFF_TOOL overrides' '
 	git config diff.tool bogus-tool &&
 	git config merge.tool bogus-tool &&
 
-	GIT_MERGE_TOOL=test-tool &&
-	export GIT_MERGE_TOOL &&
-	diff=$(git difftool --no-prompt branch) &&
-	test "$diff" = "branch" &&
-	unset GIT_MERGE_TOOL &&
-
-	GIT_MERGE_TOOL=bogus-tool &&
 	GIT_DIFF_TOOL=test-tool &&
-	export GIT_MERGE_TOOL &&
 	export GIT_DIFF_TOOL &&
 
 	diff=$(git difftool --no-prompt branch) &&