Fix flaky gclient_smoketest.

It assumed, incorrectly, that git always listens on port 20000.
It's mostly true, except when tests are ran in parallel.

Example of flake: https://logs.chromium.org/logs/infra/buildbucket/cr-buildbucket.appspot.com/8912524891032998560/+/steps/presubmit/0/stdout

R=smut

Recipe-Nontrivial-Roll: build
Change-Id: I96825175e2f899bce703443b80b71bba5953ac72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1629745
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: smut <smut@google.com>
Auto-Submit: Andrii Shyshkalov <tandrii@chromium.org>

tests/gclient_smoketest.py

@@ -1106,12 +1106,12 @@ class GClientSmokeGIT(GClientSmokeBase):
         '',
         '  # src -> src/repo15',
         '  "src/repo15": {',
-        '    "url": "git://127.0.0.1:20000/git/repo_15",',
+        '    "url": "' + self.git_base + 'repo_15",',
         '  },',
         '',
         '  # src -> src/repo16',
         '  "src/repo16": {',
-        '    "url": "git://127.0.0.1:20000/git/repo_16",',
+        '    "url": "' + self.git_base + 'repo_16",',
         '  },',
         '',
         '  # src -> src/repo2',