Stop gh re-drifting the gitconfig credential blocks

[DROOdotFOO]

Closes the last known self-rewriting file, after #17 handled the two JSON ones.

Cause

gh auth setup-git does not hand-write the credential blocks -- it goes through
git config, which has its own canonical format. Reproduced against a scratch
file:

[credential "https://github.com"]
<TAB>helper =<TRAILING SPACE>
<TAB>helper = !/opt/homebrew/bin/gh auth git-credential

The source was space-indented with no trailing space, so the two never matched
and every gh auth setup-git re-drifted ~/.gitconfig. The previous mitigation
was a comment telling you to run chezmoi apply --force.

Fix

Match git config byte-for-byte, so gh's rewrite is a no-op.

The trailing space is the catch: it cannot exist literally in the source, because
the trailing-whitespace pre-commit hook only excludes home/.*/encrypted_ and
would strip it on the next commit -- silently reintroducing the exact drift being
fixed. So it is emitted by a {{ " " }} template expression, leaving the source
file free of literal trailing whitespace.

No modify_ script here. Unlike settings.json, only these two blocks are ever
rewritten and the values are already fully managed, so there is nothing to merge
-- and INI has no jq equivalent, which would have meant git config -f against
a temp file for no benefit.

Manual testing

• Reproduced git's exact output on a scratch file to get the format right rather than guessing
• Rendered template matches git config byte-for-byte (tab + trailing space)
• grep -n " $" on the source: no literal trailing whitespace
• pre-commit run passes, and the template still renders the tab and trailing space afterwards -- the hook no longer defeats it
• Ran gh auth setup-git three times; chezmoi status ~/.gitconfig stayed clean each time
• Both helpers still resolve (git config --get-all), and gh auth status still reports logged in over https

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