project.RemoteFetch: Handle depth cases more robustly

The fetch logic for the case where depth is set and revision is a
SHA1 has several failure modes that are not handled well by the
current logic.

1) 'git fetch <SHA1>' requires git version >= 1.8.3
2) 'git fetch <SHA1>' can be prevented by a configuration option on the server.
3) 'git fetch --depth=<N> <refspec>' can fail to contain a SHA1 specified by
   the manifest.

Each of these cases cause infinite recursion when _RemoteFetch() tries to call
itself with current_branch_only=False because current_branch_only is set to
True when depth != None.

To try to prevent the infinite recursion, we set self.clone_depth to None
before the first retry of _RemoteFetch(). This will allow the Fetch to
eventually succeed in the case where clone-depth is specified in the manifest.
A user specified depth from the init command will still recurse infinitely.

In addition, never try to fetch a SHA1 directly if the git version being used
is not at least 1.8.3.

Change-Id: I802fc17878c0929cfd63fff611633c1d3b54ecd3

project.py

@@ -1897,7 +1897,7 @@ class Project(object):
 
     if not self.manifest.IsMirror:
       branch = self.revisionExpr
-      if is_sha1 and depth:
+      if is_sha1 and depth and git_require((1, 8, 3)):
         # Shallow checkout of a specific commit, fetch from that commit and not
         # the heads only as the commit might be deeper in the history.
         spec.append(branch)
@@ -1960,8 +1960,15 @@ class Project(object):
       # got what we wanted, else trigger a second run of all
       # refs.
       if not self._CheckForSha1():
+        if not depth:
+          # Avoid infinite recursion when depth is True (since depth implies
+          # current_branch_only)
           return self._RemoteFetch(name=name, current_branch_only=False,
                                    initial=False, quiet=quiet, alt_dir=alt_dir)
+        if self.clone_depth:
+          self.clone_depth = None
+          return self._RemoteFetch(name=name, current_branch_only=current_branch_only,
+                                   initial=False, quiet=quiet, alt_dir=alt_dir)
 
     return ok