Support bug prefixes when stripping away empty bug in git-cl-upload.

For instance, if the bug-prefix is set to "b/", git cl upload will offer a
line with "Bug: b/", however it won't get stripped away if it's left blank,
unlike the default "Bug:" line. This change fixes that inconsistency by
also taking the bug-prefix into account when stripping.

Change-Id: Ib6e4d18c1ff52ec77cd1422be15b1e6920332238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1528972
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Jonas Termansen <sortie@google.com>
diff --git a/tests/git_cl_test.py b/tests/git_cl_test.py
index f71a5d5..63ac18f 100755
--- a/tests/git_cl_test.py
+++ b/tests/git_cl_test.py
@@ -964,6 +964,7 @@
       if not issue:
         # Prompting to edit description on first upload.
         calls += [
+          ((['git', 'config', 'rietveld.bug-prefix'],), ''),
           ((['git', 'config', 'core.editor'],), ''),
           ((['RunEditor'],), description),
         ]