bitbake: fetch/git: Avoid clean upon failure

Currently when git fetches fail, it destroys all the existing local clone data.
For large repositories this can introduce long build delays when for example,
you just typo'd the git revision hash.

The git fetcher should be able to recover most directories so when the fetch is
for a git repo, avoid removing things unless clean is explicitly called
(e.g. a -c cleanall task).

(Bitbake rev: 1b3cd039fe19b24bd4be9a0202a98cdcbb0e9443)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/fetch2/git.py

@@ -150,6 +150,9 @@ class Git(FetchMethod):
     def supports_checksum(self, urldata):
         return False
 
+    def cleanup_upon_failure(self):
+        return False
+
     def urldata_init(self, ud, d):
         """
         init git specific variable within url data