upload-pack: make sure deepening preserves shallow roots

When "fetch --depth=N" where N exceeds the longest chain of history in
the source repo, usually we just send an "unshallow" line to the
client so full history is obtained.

When the source repo is shallow we need to make sure to "unshallow"
the current shallow point _and_ "shallow" again when the commit
reaches its shallow bottom in the source repo.

This should fix both cases: large <N> and --unshallow.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/fetch-options.txt

@@ -14,8 +14,12 @@
 	branch history. Tags for the deepened commits are not fetched.
 
 --unshallow::
-	Convert a shallow repository to a complete one, removing all
-	the limitations imposed by shallow repositories.
+	If the source repository is complete, convert a shallow
+	repository to a complete one, removing all the limitations
+	imposed by shallow repositories.
++
+If the source repository is shallow, fetch as much as possible so that
+the current repository has the same history as the source repository.
 
 ifndef::git-pull[]
 --dry-run::