Add presubmit check for git conflict markers

Errors will be reported like this:
"""""""""
Running presubmit upload checks ...

** Presubmit ERRORS **
Git conflict markers found in whitespace.txt:14 <<<<<<< HEAD

Git conflict markers found in whitespace.txt:16 =======

Git conflict markers found in whitespace.txt:18 >>>>>>> 75a85179504e59df7b3abbc09113f24f2233001b
"""""""""

Bug: skia:11098
Change-Id: I24f90ec545b797f1d782a1733b8273ce176b1012
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/345037
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
1 file changed