[github] Make branch workflow more robust (#66781)

Avoid false positives by requiring space after `/branch` command so the
action won't trigger on diffs that include filenames like
`.../BranchProbabilityInfo.cpp`.

Author: danilaml

.github/workflows/issue-release-workflow.yml

@@ -70,7 +70,7 @@ jobs:
     if: >-
       (github.repository == 'llvm/llvm-project') &&
       !startswith(github.event.comment.body, '<!--IGNORE-->') &&
-      contains(github.event.comment.body, '/branch')
+      contains(github.event.comment.body, '/branch ')
 
     steps:
       - name: Fetch LLVM sources