bitbake: fetch2/git.py: add comment in try_premirrors

The purpose of ensuring 'incremental fetch' is not easy to see from
the codes. So add comments to explain this.

(Bitbake rev: 8b890b87e30cd05ec92ed71ee3691a47b4d77253)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/fetch2/git.py

@@ -361,6 +361,11 @@ class Git(FetchMethod):
         # we need to try premirrors first as using upstream is destined to fail.
         if not trusted_network(d, ud.url):
             return True
+        # the following check is to ensure incremental fetch in downloads, this is
+        # because the premirror might be old and does not contain the new rev required,
+        # and this will cause a total removal and new clone. So if we can reach to
+        # network, we prefer upstream over premirror, though the premirror might contain
+        # the new rev.
         if os.path.exists(ud.clonedir):
             return False
         return True