convert: add a mode where mercurial_sink skips empty revisions. The getchanges function of some converter_source classes can return some false positives. I.e. they sometimes claim that a file "foo" was changed in some revision, even though its contents are still the same. convert_svn is particularly bad, but I think this can also happen with convert_cvs and, at least in theory, with mercurial_source. For regular conversions this is not really a problem - as long as getfile returns the right contents, we'll get a converted revision with the right contents. But when we use --filemap, this could lead to superfluous revisions being converted. Instead of fixing every converter_source, I decided to change mercurial_sink to work around this problem. When --filemap is used, we're interested only in revisions that touch some specific files. If a revision doesn't change any of these files, then we're not interested in it (at least for revisions with a single parent; merges are special). For mercurial_sink, we abuse this property and rollback a commit if the manifest text hasn't changed. This avoids duplicating the logic from localrepo.filecommit to detect unchanged files.

#!/bin/sh
#
# Corrupt an hg repo with two pulls.
#

# create one repo with a long history
hg init source1
cd source1
touch foo
hg add foo
for i in 1 2 3 4 5 6 7 8 9 10; do
    echo $i >> foo
    hg ci -m $i
done
cd ..

# create one repo with a shorter history
hg clone -r 0 source1 source2
cd source2
echo a >> foo
hg ci -m a
cd ..

# create a third repo to pull both other repos into it
hg init corrupted
cd corrupted
# use a hook to make the second pull start while the first one is still running
echo '[hooks]' >> .hg/hgrc
echo 'prechangegroup = sleep 5' >> .hg/hgrc

# start a pull...
hg pull ../source1 &

# ... and start another pull before the first one has finished
sleep 1
hg pull ../source2 2>/dev/null

# see the result
wait
hg verify