Allow using shallow clones as source

[mbaraban]

Sample output demonstrating a problem that this change fixes (while filtering a shallow repository):

+ echo '  - fetching'
  - fetching
+ git -C /path/to/temp/worker-13c43a54c95e3f3437439950948004a2 fetch --quiet -
-tags source mybranch
error: Could not read 698e7ab4fee66cdc158f3f89b7d5853a641d7f57
fatal: Failed to traverse parents of commit 8d5dc10f94b714e90e8516c840298de4eed7a366
error: /path/to/temp/source-4e0ccd572832548afab7d6685895556b did not send all 
necessary objects

[mlocati]

When does this happen? Isn't --update-shallow meant to be used when fetching from shallow repositories? If so, it shouldn't be needed, since the affected git-fetch command is fetching from a full mirror...

[mbaraban]

Looks like if the directory to be filtered is shallow, the mirror in temp will also be shallow. So unless --update-shallow is used in the code, it fails. Should be easy to reproduce: try filtering a directory created with

git clone --depth=2 file://`pwd`/source destination

[mlocati]

That makes sense.
Thanks!