Add optional stop revision to revlog.heads

Change repo.comit to return None on error or the new revision number on success This allows the caller to correctly record the new revision number

Add force option to repo.commit, allowing commits where no files change

Add '-' support to the commit logfile option to read message from stdin.

Add searching for named branches named branching is when you identify a head by a tag on an earlier revision. This patch adds repo.branchlookup for searching through the tree to find branch tags for heads. hg update -b tag is added to checkout based on branch tags hg heads -b is added to print the tag associated with each head

Fix bug with empty inc and exc This fixes an exception that showed up when importing patches