git-difftool: add page (#4127)

pages/common/git-difftool.md

@@ -0,0 +1,20 @@
+# git difftool
+
+> Show file changes using external diff tools. Accepts the same options and arguments as git diff.
+> More information: <https://git-scm.com/docs/git-difftool>.
+
+- List available diff tools:
+
+`git difftool --tool-help`
+
+- Set the default diff tool to meld:
+
+`git config --global diff.tool "{{meld}}"`
+
+- Use the default diff tool to show staged changes:
+
+`git difftool --staged`
+
+- Use a specific tool (opendiff) to show changes since a given commit:
+
+`git difftool --tool={{opendiff}} {{commit}}`