fix(git): don't pass extraCloneOpts to ls-remote when local clone exists

[Churro]

Changes

This PR fixes a case where git ls-remote fails during a recursive renovateRepository() call after automerge. In fetchBranchCommits(), skip injecting extraCloneOpts as -c flags when .git/HEAD already exists in localDir.

With token auth, getExtraCloneOpts() sets up http.extraHeader, which gets persisted in .git/config. Until now, the directory survived the recursive call and fetchBranchCommits added those same options again via -c, producing duplicate HTTP headers that servers reject with HTTP 400. See #42210 (comment) for details.

Context

Please select one of the following:

• This closes an existing Issue, Closes: ls-remote failing after automerge on Bitbucket – could not read Username – No such device or address #42305
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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