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

mkdir t
cd t
hg init

echo 1 > a
hg ci -qAm "first" -d "1000000 0"

hg cp a b
hg mv a c
echo 2 >> b
echo 2 >> c

hg ci -qAm "second" -d "1000000 0"

hg co -C 0

echo 0 > a
echo 1 >> a

hg ci -qAm "other" -d "1000000 0"

hg merge --debug

echo "-- b --"
cat b

echo "-- c --"
cat c