git-mergetool: add page (tldr-pages#3947)

pages/common/git-mergetool.md

@@ -0,0 +1,28 @@
+# git mergetool
+
+> Run merge conflict resolution tools to resolve merge conflicts.
+> More information: <https://git-scm.com/docs/git-mergetool>.
+
+- Launch the default merge tool to resolve conflicts:
+
+`git mergetool`
+
+- List valid merge tools:
+
+`git mergetool --tool-help`
+
+- Launch the merge tool identified by a name:
+
+`git mergetool --tool {{tool_name}}`
+
+- Don't prompt before each invocation of the merge tool:
+
+`git mergetool --no-prompt`
+
+- Explicitly use the GUI merge tool (see the `merge.guitool` config variable):
+
+`git mergetool --gui`
+
+- Explicitly use the regular merge tool (see the `merge.tool` config variable):
+
+`git mergetool --no-gui`