tags: don't crash if unable to write tag cache
This happens with hgweb in real life, if the httpd user is unable to
write in the repository directory. Another case is doing 'hg incoming'
on a repository in the filesystem owned by someone else.
help: document isodatesec and rfc3339 filters (
issue1799)
iss: make mfc*.dll and msvc*.dll optional
mfc71.dll was only needed for Python2.4
msvc*.dll is similarly optional
Do not bail installer build if not found