[release] Relax git_footers parsing to match that of Gerrit (JGit).

Port https://chromium-review.googlesource.com/c/501849/

NOTRY=true
TBR=tandrii@chromium.org
Bug: chromium:717504

Change-Id: Ia37759c615cc3ad4d2978a4589ca687a750afc46
Reviewed-on: https://chromium-review.googlesource.com/503028
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45249}

tools/release/git_recipes.py

@@ -67,9 +67,9 @@ def GetCommitMessageFooterMap(message):
   for line in lines:
     m = COMMIT_FOOTER_ENTRY_RE.match(line)
     if not m:
-      # If any single line isn't valid, the entire footer is invalid.
-      footers.clear()
-      return footers
+      # If any single line isn't valid, continue anyway for compatibility with
+      # Gerrit (which itself uses JGit for this).
+      continue
     footers[m.group(1)] = m.group(2).strip()
   return footers