Fix hg clone race with writer
Most read operations in hg don't need locks because we order reads and
writes for consistency. Clone is an exception to this as we're copying
entire file histories and could end up with more file history copied
than we have commits.
For now, make clone take a lock on the source repo. Non-hardlinked
clone should eventually be changed to use lockless pull.
+ hg init
+ hg add a
+ hg commit -m a -u a -d 0 0
+ hg init
+ hg add b
+ hg commit -m b -u b -d 0 0
+ hg pull ../a
pulling from ../a
searching for changes
warning: pulling from an unrelated repository!
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
(run 'hg update' to get a working copy)
+ hg heads
changeset: 1:9a79c33a9db3
tag: tip
user: a
date: Thu Jan 1 00:00:00 1970
summary: a
changeset: 0:01f8062b2de5
user: b
date: Thu Jan 1 00:00:00 1970
summary: b