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.

adding bar
adding foo
adding bomb
adding a.c
adding dir/a.o
adding dir/b.o
M dir/b.o
! a.c
! dir/a.o
? .hgignore
a.c: unsupported file type (type is fifo)
! a.c
# test absolute path through symlink outside repo
A f
# try symlink outside repo to file inside
abort: ../z not under root
# try cloning symlink in a subdir
1. commit a symlink
? a/b/c/demo
adding a/b/c/demo
2. clone it
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
# git symlink diff
diff --git a/a/b/c/demo b/a/b/c/demo
new file mode 120000
--- /dev/null
+++ b/a/b/c/demo
@@ -0,0 +1,1 @@
+/path/to/symlink/source
\ No newline at end of file
# import git symlink diff
applying ../sl.diff
diff --git a/a/b/c/demo b/a/b/c/demo
new file mode 120000
--- /dev/null
+++ b/a/b/c/demo
@@ -0,0 +1,1 @@
+/path/to/symlink/source
\ No newline at end of file