repo: add --show-toplevel akin to git

Simple API to make it easy to find the top of the repo client checkout
for users.  This mirrors the `git rev-parse --show-toplevel` API.

Change-Id: I0c3f98def089d0fc9ebcfa50aa3dc02091c1c273
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/312909
Reviewed-by: Xin Li <delphij@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

main.py

@@ -118,6 +118,10 @@ global_options.add_option('--time',
 global_options.add_option('--version',
                           dest='show_version', action='store_true',
                           help='display this version of repo')
+global_options.add_option('--show-toplevel',
+                          action='store_true',
+                          help='display the path of the top-level directory of '
+                               'the repo client checkout')
 global_options.add_option('--event-log',
                           dest='event_log', action='store',
                           help='filename of event log to append timeline to')
@@ -202,6 +206,9 @@ class _Repo(object):
     elif gopts.show_version:
       # Always allow global --version regardless of subcommand validity.
       name = 'version'
+    elif gopts.show_toplevel:
+      print(os.path.dirname(self.repodir))
+      return 0
     elif not name:
       # No subcommand specified, so show the help/subcommand.
       self._PrintHelp(short=True)

man/repo.1

@@ -34,6 +34,10 @@ time repo command execution
 \fB\-\-version\fR
 display this version of repo
 .TP
+\fB\-\-show\-toplevel\fR
+display the path of the top\-level directory of the
+repo client checkout
+.TP
 \fB\-\-event\-log\fR=\fI\,EVENT_LOG\/\fR
 filename of event log to append timeline to
 .TP