fix: keep git output out of the changelog increment file

When `changelog_increment_filename` is set, the action redirects the
bump command's stdout to that file. Without `--git-output-to-stderr`,
the post-bump `git commit` output (e.g. `[main abcdef] bump:
version 0.0.0 -> 0.1.0` and the `2 files changed, ...` summary)
ends up appended to the changelog file, producing a noisy artifact
that has to be hand-cleaned before it can be used as a release body.

When `changelog_increment_filename` is supplied, force
`--git-output-to-stderr` unless the user already enabled it via
`git_redirect_stderr: true`. The changelog file then contains only
the actual changelog content.

Closes #82

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bearomorphism

entrypoint.sh

@@ -95,6 +95,12 @@ if [[ $INPUT_MANUAL_VERSION ]]; then
   CZ_CMD+=("$INPUT_MANUAL_VERSION")
 fi
 if [[ $INPUT_CHANGELOG_INCREMENT_FILENAME ]]; then
+  # Avoid polluting the changelog increment file with git's commit output
+  # (e.g. "[main abcdef] bump: ..." and the file-change summary).
+  # Force git output to stderr unless the user already enabled it.
+  if [[ $INPUT_GIT_REDIRECT_STDERR != 'true' ]]; then
+    CZ_CMD+=('--git-output-to-stderr')
+  fi
   CZ_CMD+=('--changelog-to-stdout')
   echo "${CZ_CMD[@]}" ">$INPUT_CHANGELOG_INCREMENT_FILENAME"
   "${CZ_CMD[@]}" >"$INPUT_CHANGELOG_INCREMENT_FILENAME"