Make upload_revision.py do a git checkout so it can get the commit count

And make sure that the 'n' prefix gets used properly.

TBR=thakis

Bug: 1017736
Change-Id: I9d65fd468ec4d58ab7ddf3ce35160e13f518514e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879227Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709107}

tools/clang/scripts/build.py

@@ -148,7 +148,9 @@ def GetLatestLLVMCommit():
 
 
 def GetCommitCount(commit):
-  """Get the number of commits from FIRST_LLVM_COMMIT to commit."""
+  """Get the number of commits from FIRST_LLVM_COMMIT to commit.
+
+  Needs to be called from inside the git repository dir."""
   return subprocess.check_output(['git', 'rev-list', '--count',
                                   FIRST_LLVM_COMMIT + '..' + commit]).rstrip()
 

tools/clang/scripts/upload_revision.py

@@ -18,7 +18,8 @@ import shutil
 import subprocess
 import sys
 
-from build import GetCommitCount
+from build import GetCommitCount, CheckoutLLVM, LLVM_DIR
+from update import CHROMIUM_DIR
 
 # Path constants.
 THIS_DIR = os.path.dirname(__file__)
@@ -66,14 +67,18 @@ def main():
   args = parser.parse_args()
 
   clang_git_revision = args.clang_git_revision[0]
-  clang_svn_revision = GetCommitCount(clang_git_revision)
+
+  # To get the commit count, we need a checkout.
+  CheckoutLLVM(clang_git_revision, LLVM_DIR);
+  clang_svn_revision = 'n' + GetCommitCount(clang_git_revision)
   clang_sub_revision = args.clang_sub_revision
 
   # Needs shell=True on Windows due to git.bat in depot_tools.
+  os.chdir(CHROMIUM_DIR)
   git_revision = subprocess.check_output(
       ["git", "rev-parse", "origin/master"], shell=is_win).strip()
 
-  print("Making a patch for Clang n{}-{}-{}".format(
+  print("Making a patch for Clang {}-{}-{}".format(
       clang_svn_revision, clang_git_revision[:8], clang_sub_revision))
   print("Chrome revision: {}".format(git_revision))