git-squash: add page (#7361)

pages/common/git-squash.md

@@ -0,0 +1,21 @@
+# git squash
+
+> Squash multiple commits into a single commit.
+> Part of `git-extras`.
+> More information: <https://github.com/tj/git-extras/blob/master/Commands.md#git-squash>.
+
+- Merge all commits from a specific branch into the current branch as a single commit:
+
+`git squash {{source_branch}}`
+
+- Squash all commits starting with a specific commit on the current branch:
+
+`git squash {{commit}}`
+
+- Squash the `n` latest commits and commit with a message:
+
+`git squash HEAD~{{n}} "{{message}}"`
+
+- Squash the `n` latest commits and commit concatenating all individual messages:
+
+`git squash --squash-msg HEAD~{{n}}`