clone: if "url#rev" was given, update to rev

parseurl: also return the revision after the "#"; add a test

pull -r: pass the revisions as the heads argument of findincoming This can make a hg pull -r faster if the remote repo has many heads, and fixes an "abort: received changelog group is empty".

convert: fix /dev/null redirections under Windows

Test git repository conversion

hghave: detect git availability

convert: fix issue702 about GIT_DIR= construct unsupported under Windows.

convert: gitcmd wrapper for os.popen