Only remove .gitrepo on appropriate patches

Limit the removal of .gitrepo files to commits on the repo hosting
the subrepo. When using filter-branch and it encountered a merge
commit, continuing the filter on the second branch rewrites all
of those commits, despite that not being necessary for pre-commit.

If the rewritten commits were GPG signed the change in the hash
results in the inability to push to the remote repo.

If during development you were to change to a different branch
on the remote repo that contained a merge, you would not be able
to push upstream because the rewrite would not match the upstream
SHAs.

fixes issue ingydotnet#613
fixes isseu ingydotnet#602

Author: edvinassabaliauskas

lib/git-subrepo

@@ -869,7 +869,7 @@ subrepo:branch() {
   local filter=$branch
   [[ $first_gitrepo_commit ]] && filter=$first_gitrepo_commit..$branch
   FAIL=false RUN git filter-branch -f --prune-empty --tree-filter \
-    "rm -f .gitrepo" "$filter"
+    "rm -f .gitrepo" -- "$filter" --first-parent
 
   git:create-worktree "$branch"