pre-upload: Allow exec_files check to be disabled

This patch allows PRESUBMIT.cfg files to disable the check for files
incorrectly marked as executable. Of course, normally this check should
never be disabled and the sources should be fixed instead. However, in
certain repositories (e.g. firmware blob sources) we check in large
unmodified drops of vendor code. Depending on how we get those drops
(e.g. checkout from a different Git remote), they may already come with
executable permissions set, and those permissions may be wrong. There is
little point in tasking the Googler checking in this code with cleaning
up all those permissions -- they'd be incorrect with the next drop again
anyway, and with some vendors there is little hope of getting them to
clean up something like this on their end.

BUG=b:119578707
TEST=None

Change-Id: I58c0811ce2d62b967c9773eeb82c7780c3c4f7cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/repohooks/+/2333280
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Julius Werner <jwerner@chromium.org>
Commit-Queue: Julius Werner <jwerner@chromium.org>
diff --git a/pre-upload.py b/pre-upload.py
index e8b29f2..66652b5 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -1987,6 +1987,7 @@
     'project_prefix_check': _check_project_prefix,
     'filepath_chartype_check': _check_filepath_chartype,
     'cargo_clippy_check': _check_cargo_clippy,
+    'exec_files_check': _check_exec_files,
 }