Use patch.patch() when checking for patch availability.
The test is closer to what we want to know. It also makes sense when you deal with patched versions of patch() as most people do under win32.
#!/bin/sh
hg init a
cd a
touch empty1
hg add empty1
hg commit -m 'add empty1' -d '1000000 0'
touch empty2
hg add empty2
hg commit -m 'add empty2' -d '1000000 0'
hg up -C 0
touch empty3
hg add empty3
hg commit -m 'add empty3' -d '1000000 0'
hg heads
hg merge 1
# before changeset 05257fd28591, we didn't notice the
# empty file that came from rev 1.
hg status
hg commit -m merge -d '1000000 0'
hg manifest --debug tip