hgweb: set minimum number of revision to display to 1 when revcount is 0
When revcount goes to 0 nothing is displayed in hgweb. This patch sets revcount to max(revcount, 1).
http://mercurial.selenic.com/bts/issue433
$ hg init
$ echo a > a
$ hg commit -Ama
adding a
$ hg parents -r 0 doesnotexist
abort: 'doesnotexist' not found in manifest!
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