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

# 0-2-4
#  \ \ \
#   1-3-5
#
# rename in #1, content change in #4.

hg init t
cd t

touch 1
touch 2
hg commit -Am init -d "0 0"  # 0

hg rename 1 1a
hg commit -m rename -d "0 0" # 1

hg co -C 0
echo unrelated >> 2
hg ci -m unrelated1 -d "0 0"  # 2

hg merge --debug 1
hg ci -m merge1 -d "0 0" # 3

hg co -C 2
echo hello >> 1
hg ci -m unrelated2 -d "0 0" # 4

hg co -C 3
hg merge -y --debug 4

hg co -C 4
hg merge -y --debug 3