Use FS_CHMOD_FILE rather than an explicit 0644 in copy_dir() and _copy_dir().

This occurs when we can't copy a file. We chmod it and try again.

see #22704.

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Core Updates: Verify that all files we're about to copy are actually writable, when using the direct transport.

Once we begin to copy core files, all bets are off. This allows us to fail early when we'll otherwise need to stop midway through due to permissions issues, which is a particularly bad problem when only some files have permissions issues.

see #22704.

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3.7-beta2-25795

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In show_message(), only print error data if it is a string.

see #25576, [25775].

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Only add error data to the files_not_writable WP_Error if the install already has [25775] and [25796] applied.

see #22704.

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Avoid our pre-flight writable checks if get_core_checksums() doesn't exist yet.

Thus, it will not work for 3.6 => 3.7, only 3.7+.

see #22704.

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Remove accidental debug cruft in [25780]. see #22704.

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Use correct variable. see #22704.

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Significantly simplify get_core_checksums(), as the caching and chunking was causing too much grief.

Make sure we only do our pre-flight is_writable check when the file exists.

see #18201. see #22704.

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Twenty Fourteen: better logic for featured image HTML output, and add fallback message for focusable anchor elements, for accessibility. Fixes #25325.
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Twenty Fourteen: link hover improvements, props sabreuse. See #25054.
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