setup: make error message for missing Python headers more helpful

obsolete: ensure all markers have a date Obsolescence creates a sparse DAG mostly composed of a lot of small independent chain of markers. Date is the only simple and "reliable" way to sort them. The existence of a date is now enforced at creation time as I'm more and more convinced that date will have a key role in obsolescence markers exchange.

zsh_completion: make use of `debuglabelcomplete` command Use `debuglabelcomplete` command when populating labels list, instead of calling `hg` three times: for branches, bookmarks and tags. Do not pass string being completed to `hg debuglabelcomplete` (as `$words[$CURRENT]`), since it breaks `_hg_revrange` completion (`--rev 2.5:2.5.<Tab>`) for no apparent benefit. Also complete `hg view` with labels, not just tags, and drop unused `_hg_tags`.

dirs: use mutable strings internally perfdirs results for a working dir with 170,000 files: Python 638 msec C 244 C+int 192 C+int+str 168 In the large repo above, the nearly 0.5 second time improvement is visible in commands like "hg add" and "hg update". hg add Python 1100 msec C+int+str 600 hg update (with nothing to do) Python 2800 msec C+int+str 2240

dirs: use mutable integers internally These integers are not visible to Python code, so this is safe. perfdirs results for a working dir with 170,000 files: Python 638 msec C 244 C+int 192

scmutil: rewrite dirs in C, use if available This is over twice as fast as the Python dirs code. Upcoming changes will nearly double its speed again. perfdirs results for a working dir with 170,000 files: Python 638 msec C 244

scmutil: use new dirs class in dirstate and context The multiset-of-directories code was open coded in each of these modules; this change gets rid of the duplication.