upload: Clarify partial sync message on hook failure

Demote the partial sync warning from error to info and rephrase it to
be a tip rather than an error. This prevents users from thinking that
a partial sync is the cause of their hook failures when it is often a linting failure.

The message now suggests that a full sync might help if there are
cross-project dependencies, instead of implying it will fix any issue.

Change-Id: I5d8c52b53ac315aa9f145ed069798bf201fa0815
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/574262
Tested-by: Marty Heavey <mheavey@google.com>
Commit-Queue: Marty Heavey <mheavey@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

subcmds/upload.py

@@ -803,9 +803,10 @@ Gerrit Code Review:  https://www.gerritcodereview.com/
                 project_list=pending_proj_names, worktree_list=pending_worktrees
             ):
                 if LocalSyncState(manifest).IsPartiallySynced():
-                    logger.error(
-                        "Partially synced tree detected. Syncing all projects "
-                        "may resolve issues you're seeing."
+                    logger.info(
+                        "Tip: A partially synced tree was detected. "
+                        "If this failure involves cross-project dependencies, "
+                        "a full `repo sync` might help."
                     )
                 ret = 1
         if ret: