main: improve launcher update messaging wrt system installs

Some users get repo from their distro (e.g. /usr/bin/repo), so the
suggestion to copy over top of it makes people uneasy, if it's even
possible in the first place.

Bug: https://crbug.com/gerrit/12335
Change-Id: I9a0c83d6ba0f466fa8e6d61f674ee13396f9a968
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256893
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>

main.py

@@ -348,12 +348,20 @@ repo: error:
     sys.exit(1)
 
   if exp > ver:
-    print("""
-... A new version of repo (%s) is available.
+    print('\n... A new version of repo (%s) is available.' % (exp_str,),
+          file=sys.stderr)
+    if os.access(repo_path, os.W_OK):
+      print("""\
 ... You should upgrade soon:
-
     cp %s %s
-""" % (exp_str, WrapperPath(), repo_path), file=sys.stderr)
+""" % (WrapperPath(), repo_path), file=sys.stderr)
+    else:
+      print("""\
+... New version is available at: %s
+... The launcher is run from: %s
+!!! The launcher is not writable.  Please talk to your sysadmin or distro
+!!! to get an update installed.
+""" % (WrapperPath(), repo_path), file=sys.stderr)
 
 
 def _CheckRepoDir(repo_dir):