Git checkout creates duplicate branches in localized environments

[YearsAlso]

Description:
The current switchToMaybeExistingBranch method determines branch checkout success by parsing git checkout command's stderr output. However, since Git client messages vary by system language locale, in non-English environments (e.g., Chinese, Japanese), the stderr.includes() string matching may fail, causing:

1. False triggering of git checkout -b to recreate existing branches
2. Potential error: fatal: a branch named 'xxx' already exists

Reproduction Steps:

1. Set OS/Git language to non-English (e.g., export LANG=zh_CN.UTF-8)
2. Call switchToMaybeExistingBranch with an existing branch name
3. Observe git checkout -b being incorrectly triggered with error

Affected Code:
https://github.com/un-ts/changesets-gitlab/blob/main/src/git-utils.ts#L37-L48

Environment:

• OS: All localization-supported systems
• Git Versions: All versions affected

[JounQin]

Do you have any idea how to fix this issue? Personally I'm not running this cli locally.

[JounQin]

OK, I think we can just pass env: { LANG: 'en_US.UTF-8' } unconditionally into exec.

@YearsAlso Do you want to raise a PR for it?