Various 'reset' optimizations and clean-ups, followed by a change
to allow "git reset" to work even on an unborn branch.
* mz/reset-misc:
reset: update documentation to require only tree-ish with paths
reset [--mixed]: use diff-based reset whether or not pathspec was given
reset: allow reset on unborn branch
reset $sha1 $pathspec: require $sha1 only to be treeish
reset.c: inline update_index_refresh()
reset.c: finish entire cmd_reset() whether or not pathspec is given
reset [--mixed]: only write index file once
reset.c: move lock, write and commit out of update_index_refresh()
reset.c: move update_index_refresh() call out of read_from_tree()
reset.c: replace switch by if-else
reset: avoid redundant error message
reset --keep: only write index file once
reset.c: share call to die_if_unmerged_cache()
reset.c: extract function for updating {ORIG_,}HEAD
reset.c: remove unnecessary variable 'i'
reset.c: extract function for parsing arguments
reset: don't allow "git reset -- $pathspec" in bare repo
reset.c: pass pathspec around instead of (prefix, argv) pair
reset $pathspec: exit with code 0 if successful
reset $pathspec: no need to discard index
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