Improve `git llvm push` to suggest `git pull` when applying patch fails

llvm/utils/git-svn/git-llvm

 def first_dirname(d):
     while True:
         (head, tail) = os.path.split(d)
         if not head or head == '/':
             return tail
         d = head
 
 
-def shell(cmd, strip=True, cwd=None, stdin=None):
+def shell(cmd, strip=True, cwd=None, stdin=None, die_on_failure=True):
     log_verbose('Running: %s' % ' '.join(cmd))
 
     start = time.time()
     p = subprocess.Popen(cmd, cwd=cwd, stdout=subprocess.PIPE,
                          stderr=subprocess.PIPE, stdin=subprocess.PIPE)
     stdout, stderr = p.communicate(input=stdin)
     elapsed = time.time() - start
 
     log_verbose('Command took %0.1fs' % elapsed)
 
     if p.returncode == 0:
         if stderr:
             eprint('`%s` printed to stderr:' % ' '.join(cmd))
             eprint(stderr.rstrip())
         if strip:
             stdout = stdout.rstrip('\r\n')
         return stdout
-    eprint('`%s` returned %s' % (' '.join(cmd), p.returncode))
+    err_msg = '`%s` returned %s' % (' '.join(cmd), p.returncode)
+    eprint(err_msg)
     if stderr:
         eprint(stderr.rstrip())
+    if die_on_failure:
         sys.exit(2)
+    raise RuntimeError(err_msg)
 
 
 def git(*cmd, **kwargs):
     return shell(['git'] + list(cmd), kwargs.get('strip', True))
 
 
 def svn(cwd, *cmd, **kwargs):
     # TODO: Better way to do default arg when we have *cmd?
if status:
         die("Can't push git rev %s because svn status is not empty:\n%s" %
             (rev, status))
 
     for sr in subrepos:
         diff = git('show', '--binary', rev, '--', sr, strip=False)
         svn_sr_path = os.path.join(svn_repo, GIT_TO_SVN_DIR[sr])
         # git is the only thing that can handle its own patches...
         log_verbose('Apply patch: %s' % diff)
-        shell(['git', 'apply', '-p2', '-'], cwd=svn_sr_path, stdin=diff)
+        try:
+            shell(['git', 'apply', '-p2', '-'], cwd=svn_sr_path, stdin=diff,
+                  die_on_failure=False)
+        except RuntimeError as e:
+            eprint("Patch doesn't apply: maybe you should try `git pull -r` "
+                   "first?")
+            sys.exit(2)
 
     status_lines = svn(svn_repo, 'status').split('\n')
 
     for l in (l for l in status_lines if l.startswith('?')):
         svn(svn_repo, 'add', l[1:].strip())
     for l in (l for l in status_lines if l.startswith('!')):
         svn(svn_repo, 'remove', l[1:].strip())