Revert of git_cache.py: Clobber git repos if the config is corrupt (patchset #4 id:60001 of https://codereview.chromium.org/2031773002/ )

Reason for revert:
Suspected to break builds: http://crbug.com/618124

Original issue's description:
> git_cache.py: Clobber git repos if the config is corrupt
>
> See https://build.chromium.org/p/tryserver.chromium.win/builders/win10_chromium_x64_rel_ng_exp/builds/1801/steps/bot_update/logs/stdio
> [Mirror] https://luci-logdog.appspot.com/v/?s=chromium%2Fbb%2Ftryserver.chromium.win%2Fwin10_chromium_x64_rel_ng_exp%2F1801%2F%2B%2Frecipes%2Fsteps%2Fbot_update%2F0%2Fstdout
>
> Sometimes repos become corrupt and bot_update fails spectacularly.  This usually happens due to the process being interrupted during a fetch. Git
> cache should be able to recover from this by clobbering.
>
> BUG=
>
> Committed: b48b126ffd

TBR=estaab@chromium.org,phajdan.jr@chromium.org,hinoka@google.com,hinoka@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=

Review-Url: https://codereview.chromium.org/2049463003

git_cache.py

@@ -38,7 +38,7 @@ except NameError:
 class LockError(Exception):
   pass
 
-class ClobberNeeded(Exception):
+class RefsHeadsFailedToFetch(Exception):
   pass
 
 class Lockfile(object):
@@ -246,11 +246,7 @@ class Mirror(object):
       cwd = self.mirror_path
 
     # Don't run git-gc in a daemon.  Bad things can happen if it gets killed.
-    try:
-      self.RunGit(['config', 'gc.autodetach', '0'], cwd=cwd)
-    except subprocess.CalledProcessError:
-      # Hard error, need to clobber.
-      raise ClobberNeeded()
+    self.RunGit(['config', 'gc.autodetach', '0'], cwd=cwd)
 
     # Don't combine pack files into one big pack file.  It's really slow for
     # repositories, and there's no way to track progress and make sure it's
@@ -411,7 +407,7 @@ class Mirror(object):
         self.RunGit(fetch_cmd + [spec], cwd=rundir, retry=True)
       except subprocess.CalledProcessError:
         if spec == '+refs/heads/*:refs/heads/*':
-          raise ClobberNeeded()  # Corrupted cache.
+          raise RefsHeadsFailedToFetch
         logging.warn('Fetch of %s failed' % spec)
 
   def populate(self, depth=None, shallow=False, bootstrap=False,
@@ -430,7 +426,7 @@ class Mirror(object):
       tempdir = self._ensure_bootstrapped(depth, bootstrap)
       rundir = tempdir or self.mirror_path
       self._fetch(rundir, verbose, depth)
-    except ClobberNeeded:
+    except RefsHeadsFailedToFetch:
       # This is a major failure, we need to clean and force a bootstrap.
       gclient_utils.rmtree(rundir)
       self.print(GIT_CACHE_CORRUPT_MESSAGE)