refactor(delete-merged-branches): rm -n1 from xargs, fmt pipes

[Rudxain]

• reverts git-delete-merged-branches: replace xargs -r option #654; see this
• remove -n1, because branch -d is variadic (IDK since when)
• format pipeline (split accross lines)

[spacewander]

Thank you for the PR!

However, xargs -r is a GNU extension and is not available on all platforms (e.g. older macOS, BSD, Solaris). While modern macOS xargs might support it as a no-op compatibility flag, strictly conforming POSIX systems or older BSDs will fail.

Since git-extras aims for broad portability, relying on xargs -r might be risky.

The previous if [ -n "$branches" ] block, while verbose, is fully portable.

If you want to avoid the if block, another portable way (though slightly more complex to read) used in bin/git-sed is to check for -r support dynamically, but that seems overkill here.

Removing -n 1 is a good improvement for performance!