Expand git-diff page

Added:
  * `—names-only` option.
  * `—summary` option.

pages/common/git-diff.md

@@ -4,4 +4,12 @@
 
 - Show changes to tracked files
 
-`git diff {{PATHSPEC}}`
+`git diff {{PATHSPEC}}`
+
+- Show only names of changed files.
+
+`git diff --names-only {{PATHSPEC}}`
+
+- Output a condensed summary of extended header information.
+
+`git diff --summary {{PATHSPEC}}`