phases: move root phase assignment to it's own function
This moves the initial root phase assignment to it's own function. Future
patches which make phase calculations lazy will use this function to pre-fill
certain phases which can be deduced from the roots.
$ hg init
$ echo 0 > a
$ echo 0 > b
$ hg ci -A -m m
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
$ 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 7040230c159c
[1]
$ hg cat -r 1 b
1
Test fileset
$ echo 3 > c
$ hg ci -Am addmore c
$ hg cat 'set:not(b) or a'
3
$ hg cat 'set:c or b'
1
3
$ mkdir tmp
$ hg cat --output tmp/HH_%H c
$ hg cat --output tmp/RR_%R c
$ hg cat --output tmp/h_%h c
$ hg cat --output tmp/r_%r c
$ hg cat --output tmp/%s_s c
$ hg cat --output tmp/%d%%_d c
$ hg cat --output tmp/%p_p c
$ hg log -r . --template "{rev}: {node|short}\n"
2: 45116003780e
$ find tmp -type f | sort
tmp/.%_d
tmp/HH_45116003780e3678b333fb2c99fa7d559c8457e9
tmp/RR_2
tmp/c_p
tmp/c_s
tmp/h_45116003780e
tmp/r_2