vcs: don't fail when git remote set-head fails

There's valid use-cases for setups where set-head --auto fails. This
happens when building an app from a gitlab CI checkout where no remote
tracking branches are setup.

This isn't really a fatal error. When a remote HEAD exists we'll
continue setting it and if none exists and something requires this being
set up later on (either a build script or fdroid checkupdates) then
we'll fail later on with "origin/HEAD not being known to git".
By not failing early we allow the majority of use-cases that don't need
a remote HEAD to continue with just a warning.

The setup in which this can be reproduced is as follows:
(This is roughly what gitlab runner does when setting up a git checkout
for CI)
- mkdir test && cd test
- git init
- git remote add https://gitlab.com/Bubu/fdroidclassic.git
- git fetch --all
- git checkout db0d2a9a5d1d89101a344169013ac5d518185f31
- mkdir nested_repo && cd nested_repo
- git clone .. .
- git remote set-head origin --auto
  > error: Cannot determine remote HEAD

fdroidserver/common.py

@@ -994,12 +994,14 @@ class vcs_git(vcs):
                 if p.returncode != 0:
                     lines = p.output.splitlines()
                     if 'Multiple remote HEAD branches' not in lines[0]:
-                        raise VCSException(_("Git remote set-head failed"), p.output)
-                    branch = lines[1].split(' ')[-1]
-                    p2 = FDroidPopen(['git', 'remote', 'set-head', 'origin', '--', branch],
-                                     cwd=self.local, output=False)
-                    if p2.returncode != 0:
-                        raise VCSException(_("Git remote set-head failed"), p.output + '\n' + p2.output)
+                        logging.warning(_("Git remote set-head failed: \"%s\"") % p.output.strip())
+                    else:
+                        branch = lines[1].split(' ')[-1]
+                        p2 = FDroidPopen(['git', 'remote', 'set-head', 'origin', '--', branch],
+                                         cwd=self.local, output=False)
+                        if p2.returncode != 0:
+                            logging.warning(_("Git remote set-head failed: \"%s\"")
+                                            % p.output.strip() + '\n' + p2.output.strip())
                 self.refreshed = True
         # origin/HEAD is the HEAD of the remote, e.g. the "default branch" on
         # a github repo. Most of the time this is the same as origin/master.