mergetool: use get_merge_tool function

In git-mergetool, the logic for getting which merge tool to use is
duplicated in git-mergetool--lib, except for the fact that it needs to
know whether the tool was guessed or not.

Rewrite `get_merge_tool` to return whether or not the tool was guessed
through the return code and make git-mergetool call this function
instead of duplicating the logic. Note that 1 was chosen to be the
return code of when a tool is guessed because it seems like a slightly
more abnormal condition than getting a tool that's explicitly specified
but this is completely arbitrary.

Also, let `$GIT_MERGETOOL_GUI` be set to determine whether or not the
guitool will be selected.

This change is not completely backwards compatible as there may be
external users of git-mergetool--lib. However, only one user,
git-diffall[1], was found from searching GitHub and Google, and this
tool is superseded by `git difftool --dir-diff` anyway. It seems very
unlikely that there exists an external caller that would take into
account the return code of `get_merge_tool` as it would always return 0
before this change so this change probably does not affect any external
users.

[1]: https://github.com/thenigan/git-diffall

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 12 insertions, 11 deletions

diff --git a/Documentation/git-mergetool--lib.txt b/Documentation/git-mergetool--lib.txt
index 055550b2bc..4da9d24096 100644
--- a/Documentation/git-mergetool--lib.txt
+++ b/Documentation/git-mergetool--lib.txt
@@ -28,7 +28,9 @@ to define the operation mode for the functions listed below.
 FUNCTIONS
 ---------
 get_merge_tool::
-	returns a merge tool.
+	returns a merge tool. the return code is 1 if we returned a guessed
+	merge tool, else 0. '$GIT_MERGETOOL_GUI' may be set to 'true' to
+	search for the appropriate guitool.
 
 get_merge_tool_cmd::
 	returns the custom command for a merge tool.
diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
index 7bfb6737df..46af3e60b7 100755
--- a/git-difftool--helper.sh
+++ b/git-difftool--helper.sh
@@ -71,7 +71,7 @@ then
 	then
 		merge_tool="$GIT_DIFF_TOOL"
 	else
-		merge_tool="$(get_merge_tool)" || exit
+		merge_tool="$(get_merge_tool)"
 	fi
 fi
 
diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 83bf52494c..b928179a2e 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -403,14 +403,17 @@ get_merge_tool_path () {
 }
 
 get_merge_tool () {
+	is_guessed=false
 	# Check if a merge tool has been configured
-	merge_tool=$(get_configured_merge_tool)
+	merge_tool=$(get_configured_merge_tool $GIT_MERGETOOL_GUI)
 	# Try to guess an appropriate merge tool if no tool has been set.
 	if test -z "$merge_tool"
 	then
 		merge_tool=$(guess_merge_tool) || exit
+		is_guessed=true
 	fi
 	echo "$merge_tool"
+	test "$is_guessed" = false
 }
 
 mergetool_find_win32_cmd () {
diff --git a/git-mergetool.sh b/git-mergetool.sh
index 01b9ad59b2..88fa6a914a 100755
--- a/git-mergetool.sh
+++ b/git-mergetool.sh
@@ -389,7 +389,7 @@ print_noop_and_exit () {
 
 main () {
 	prompt=$(git config --bool mergetool.prompt)
-	gui_tool=false
+	GIT_MERGETOOL_GUI=false
 	guessed_merge_tool=false
 	orderfile=
 
@@ -416,10 +416,10 @@ main () {
 			esac
 			;;
 		--no-gui)
-			gui_tool=false
+			GIT_MERGETOOL_GUI=false
 			;;
 		-g|--gui)
-			gui_tool=true
+			GIT_MERGETOOL_GUI=true
 			;;
 		-y|--no-prompt)
 			prompt=false
@@ -449,12 +449,8 @@ main () {
 
 	if test -z "$merge_tool"
 	then
-		# Check if a merge tool has been configured
-		merge_tool=$(get_configured_merge_tool $gui_tool)
-		# Try to guess an appropriate merge tool if no tool has been set.
-		if test -z "$merge_tool"
+		if ! merge_tool=$(get_merge_tool)
 		then
-			merge_tool=$(guess_merge_tool) || exit
 			guessed_merge_tool=true
 		fi
 	fi