Crash on too many changed files

Git-secrets failed a merge from upstream master, because it found some secrets in the upstream commits. Then it didn't allow me continuing the merge because 22680 modified files seems to be too many:
```
$ git merge --continue
/usr/local/bin/git-secrets: line 109: /usr/local/Cellar/git/2.17.0/libexec/git-core/git: Argument list too long
```

This fixed it for me:
```
$ git commit --no-verify
```

Start at the git-secrets script around line 109 and reproduce the failure with a merge containing many changed files. Trace how git merge --continue is invoked and verify that the command no longer fails with “Argument list too long” while secret scanning and merge continuation still work.

Written by the indexing model from the issue text.