tags: support 'instant' tag retrieval (
issue548)
- modify _readtagcache() and _writetagcache() to read/write tag->node
mapping for global tags
- if (and only if) tip unchanged, use that cached mapping to avoid
reading any revisions of .hgtags
- change so tag names are UTF-8 in memory in tags.py, and converted to
local encoding as late as possible (in localrepository._findtags())
{header}
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