zsh_completion: add '--partial' flag to completions for 'import' (issue5618)
When using the 'import' command running ZSH together with its
Mercurial-specific completions the flag '--partial' (introduced in Mercurial
3.1, 2014-08-01) is not offered as completion option.
This patch adds it to the list of completions for the 'import' command.
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