git-write-tree: add page (#11584)

* git-write-tree: add page

---------

Co-authored-by: Lena <126529524+acuteenvy@users.noreply.github.com>

pages/common/git-write-tree.md

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+# git write-tree
+
+> Low level utility to create a tree object from the current index.
+> More information: <https://git-scm.com/docs/git-write-tree>.
+
+- Create a tree object from the current index:
+
+`git write-tree`
+
+- Create a tree object without checking whether objects referenced by the directory exist in the object database:
+
+`git write-tree --missing-ok`
+
+- Create a tree object that represents a subdirectory (used to write the tree object for a subproject in the named subdirectory):
+
+`git write-tree --prefix {{subdirectory}}/`