[PATCH v3] mergetool: add automerge configuration

[Felipe Contreras <felipe.contreras@gmail.com>]

It doesn't make sense to display easily-solvable conflicts in the
different views of all mergetools.

Only the chunks that warrant conflict markers should be displayed.

In order to unobtrusively do this, add a new configuration:
mergetool.autoMerge.

See Seth House's blog post [1] for the idea, and the rationale.

[1] https://www.eseth.org/2020/mergetools.html

Original-idea-by: Seth House <seth@eseth.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---
 Documentation/config/mergetool.txt |  3 +++
 git-mergetool.sh                   | 10 ++++++++++
 t/t7610-mergetool.sh               | 18 ++++++++++++++++++
 3 files changed, 31 insertions(+)

diff --git a/Documentation/config/mergetool.txt b/Documentation/config/mergetool.txt
index 16a27443a3..43af7a96f9 100644
--- a/Documentation/config/mergetool.txt
+++ b/Documentation/config/mergetool.txt
@@ -61,3 +61,6 @@ mergetool.writeToTemp::
 
 mergetool.prompt::
 	Prompt before each invocation of the merge resolution program.
+
+mergetool.autoMerge::
+	Automatically resolve conflicts that don't require user intervention.
diff --git a/git-mergetool.sh b/git-mergetool.sh
index e3f6d543fb..6e86d3b492 100755
--- a/git-mergetool.sh
+++ b/git-mergetool.sh
@@ -274,6 +274,7 @@ merge_file () {
 		BASE=${BASE##*/}
 	fi
 
+	DIFF3="$MERGETOOL_TMPDIR/${BASE}_DIFF3_$$$ext"
 	BACKUP="$MERGETOOL_TMPDIR/${BASE}_BACKUP_$$$ext"
 	LOCAL="$MERGETOOL_TMPDIR/${BASE}_LOCAL_$$$ext"
 	REMOTE="$MERGETOOL_TMPDIR/${BASE}_REMOTE_$$$ext"
@@ -322,6 +323,15 @@ merge_file () {
 	checkout_staged_file 2 "$MERGED" "$LOCAL"
 	checkout_staged_file 3 "$MERGED" "$REMOTE"
 
+	if test "$(git config --bool mergetool.autoMerge)" = "true"
+	then
+		git merge-file --diff3 -q -p "$LOCAL" "$BASE" "$REMOTE" >"$DIFF3"
+		sed -e '/^<<<<<<< /,/^||||||| /d' -e '/^=======\r\?$/,/^>>>>>>> /d' "$DIFF3" >"$BASE"
+		sed -e '/^||||||| /,/^>>>>>>> /d' -e '/^<<<<<<< /d' "$DIFF3" >"$LOCAL"
+		sed -e '/^<<<<<<< /,/^=======\r\?$/d' -e '/^>>>>>>> /d' "$DIFF3" >"$REMOTE"
+		rm -- "$DIFF3"
+	fi
+
 	if test -z "$local_mode" || test -z "$remote_mode"
 	then
 		echo "Deleted merge conflict for '$MERGED':"
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 70afdd06fa..b75c91199b 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -828,4 +828,22 @@ test_expect_success 'mergetool -Oorder-file is honored' '
 	test_cmp expect actual
 '
 
+test_expect_success 'mergetool automerge' '
+	test_config mergetool.automerge true &&
+	test_when_finished "git reset --hard" &&
+	git checkout -b test${test_count}_b master &&
+	echo -e "base\n\na" >file1 &&
+	git commit -a -m "base" &&
+	echo -e "base\n\nc" >file1 &&
+	git commit -a -m "remote update" &&
+	git checkout -b test${test_count}_a HEAD~ &&
+	echo -e "local\n\nb" >file1 &&
+	git commit -a -m "local update" &&
+	test_must_fail git merge test${test_count}_b &&
+	yes "" | git mergetool file1 &&
+	echo -e "local\n\nc" >expect &&
+	test_cmp expect file1 &&
+	git commit -m "test resolved with mergetool"
+'
+
 test_done
-- 
2.30.0.rc0