repo: allow REPO_REV to be an env var

We do this for REPO_URL already.

Bug: https://crbug.com/gerrit/10233
Change-Id: I53410645474b00d900467c96fa5d8446f3a607d3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253552
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>
(cherry picked from commit 563f1a651298eaa3616f92c3cd7b264fe5442379)

repo

@@ -16,7 +16,9 @@ import os
 REPO_URL = os.environ.get('REPO_URL', None)
 if not REPO_URL:
   REPO_URL = 'https://gerrit.googlesource.com/git-repo'
-REPO_REV = 'repo-1'
+REPO_REV = os.environ.get('REPO_REV')
+if not REPO_REV:
+  REPO_REV = 'repo-1'
 
 # Copyright (C) 2008 Google Inc.
 #
@@ -235,10 +237,10 @@ group.add_option('--no-tags',
 group = init_optparse.add_option_group('repo Version options')
 group.add_option('--repo-url',
                  dest='repo_url',
-                 help='repo repository location', metavar='URL')
+                 help='repo repository location ($REPO_URL)', metavar='URL')
 group.add_option('--repo-branch',
                  dest='repo_branch',
-                 help='repo branch or revision', metavar='REVISION')
+                 help='repo branch or revision ($REPO_REV)', metavar='REVISION')
 group.add_option('--no-repo-verify',
                  dest='no_repo_verify', action='store_true',
                  help='do not verify repo source code')