copies: rearrange all value comparison conditional To properly handle the newly tested case (chaining of merges) we will need to detect more accurately when an actualy merging of the copy information (and superseed the two existing data). Before starting to do so, we need to reorganise the values comparison to introduce different conditional branches when such actual merging is needed/detected. To avoid mixing too many change in this complicated code, we do the reorganisation before adding the "overwrite detection" logic in the next changesets. Differential Revision: https://phab.mercurial-scm.org/D9612

test-copies: introduce merge chains test for the P/Q merges This is similar to the chaining test we have for the A/E merges. The current result of this tests is wrong, as for the other test of the same familly. This will be fixed by a later changesets. Differential Revision: https://phab.mercurial-scm.org/D10059

test-copies: add a case involving the `b` and a new `r` branch That case involve a branch overwriting copies information from the other one. It is similar to the `mBFm` / `mFBm` case except this case actual content merge is involved too. Differential Revision: https://phab.mercurial-scm.org/D10058

test-copies: introduce case combining the `p` and `q` branch That case involves conflicting copies information from each branch. It is similar to the `mAEm` / `mEAm` case except this case actual content merge is involved too. Differential Revision: https://phab.mercurial-scm.org/D10057

test-copies: add a `q` branch similar to the `e` but on the new files This branch will be used to create merge case that mirror ones involving the `e` branch. Differential Revision: https://phab.mercurial-scm.org/D10056

test-copies: add a `p` branch similar to the `a` but on the new files This branch will be used to create merge case that mirror ones involving the `a` branch. Differential Revision: https://phab.mercurial-scm.org/D10055