revlog.revision(): minor cleanup
Rename some variables, making the name more obvious (in particular "cache" was
actually two different variable.
Move code around, moving the index preloading before the deltachain computation,
without that index preloading was useless (everything was read in deltachain).
$ mkdir t
$ cd t
$ hg init
$ echo 0 > a
$ echo 0 > b
$ hg ci -A -m m -d "1000000 0"
adding a
adding b
$ hg rm a
$ hg cat a
0
$ hg cat --decode a # more tests in test-encode
0
$ echo 1 > b
$ hg ci -m m -d "1000000 0"
$ echo 2 > b
$ hg cat -r 0 a
0
$ hg cat -r 0 b
0
$ hg cat -r 1 a
a: No such file in rev 03f6b0774996
$ hg cat -r 1 b
1