Use depth flag when fetching

Currently, we only use the depth flag when cloning.  The result is that when
new project history has merges, the entire history of the merged branch is
brought in and the project becomes unshallow very quickly.  --depth and
clone-depth are often used to save on space, not just network load, so this
seems less than ideal.

This change uses --depth on every fetch (when the user has depth specified),
not just the initial clone.  The result is that the given project stays
consistently shallow as opposed to growing over time, especially when merges
are involved.

Change-Id: Iac706cfdad4a555c72f9d9f1119195d38d91df12

project.py

@@ -1815,9 +1815,7 @@ class Project(object):
 
     cmd = ['fetch']
 
-    # The --depth option only affects the initial fetch; after that we'll do
-    # full fetches of changes.
-    if depth and initial:
+    if depth:
       cmd.append('--depth=%s' % depth)
 
     if quiet: