Commands like hg help has a side effect of loading every extension. If
pygments is not installed, loading the highlight could fail immediately
with chg or HGDEMANDIMPORT=disable.
This patch special handles the ImportError so import highlight from hgext
does not raise an exception if pygments is missing. That makes the following
tests pass with --chg and without pygments:
- test-i18n.t
- test-help.t
- test-bad-extension.t
- test-convert-bzr-directories.t
- test-run-tests.t
- test-extdiff.t