git-summary: add page (#6065)

pages/common/git-summary.md

@@ -0,0 +1,21 @@
+# git summary
+
+> Display information about a Git repository.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-summary>.
+
+- Display data about a Git repository:
+
+`git summary`
+
+- Display data about a Git repository since a commit-ish:
+
+`git summary {{commit|branch_name|tag_name}}`
+
+- Display data about a Git repository, merging committers using different emails into 1 statistic for each author:
+
+`git summary --dedup-by-email`
+
+- Display data about a Git repository, showing the number of lines modified by each contributer:
+
+`git summary --line`