fix appendfile problem on macos.
when i open file with mode 'a+' on linux, seek position is 0.
on macos seek position is end of file instead. not documented.
Implementation of per-user .hgignore.
Reference: http://www.selenic.com/mercurial/bts/
issue166
If the [ui] section of .hgrc contains keys like "ignore" or
"ignore.something", the values corresponding to these keys are
treated as per-user hgignore files. These hgignore files apply to all
repositories used by that user.
Calling revlog.addgroup with an empty changegroup now raises RevlogError.
The empty changegroup can be caused by remote servers dying soon after
findincoming, and further code in pull assumes (correctly) that there are
new changesets.