tests: unify test-extension

tests: unify test-execute-bit

tests: unify test-empty-group

merge with stable

convert: better grouping of command line flags in help We normally put related command line flags after one another.

convert: help string cleanups

convert: better quoting in help text

convert: show example splice, author, and branch map entries in help Also document that - empty lines are skipped and comment are supported in author map - whitespace is not allowed in branch map entries since we split on it when parsing the file

hgweb: support very simple caching model (issue1845)

tests: extend get-with-headers to support cache testing