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simplemerge: stop merging file flags

Authored by martinvonz.

Description

simplemerge: stop merging file flags

As 384df4db6520 (merge: merge file flags together with file content,
2013-01-09) explains, we shouldn't do a 3-way merge of the
symlink. However, since 84614212ae39 (flags: actually merge flags in
simplemerge, 2020-05-16), we do that in
simplemerge.simplemerge(). What's more, the merging of the
executable flag there isn't actually necessary; it was made a no-op by
the very next commit, i.e. 4234c9af515d (flags: read flag from
dirstate/disk for workingcopyctx (issue5743), 2020-05-16).

I found the overall flag-merging code (not the bit in
simplemerge.py) very hard to follow, but I think I now finally
understand how it works. mergestate.resolve() calculates the merged
file flags and sets them on the local side of the merge (confusingly
by calling _restore_backup()). Then it calls
filemerge.filemerge(), which in turn calls
simplemerge.simplemerge() (if premerge is enabled). That means that
the flags on the local side fcs.flags() are already correct when the
flag-merging code in simplemerge.simplemerge() runs. Interestingly,
that code still works when the local side already has the merged
value, it just doesn't change the value. Here's a truth table to
explain why:

BLOMCAR
0000000
0011111
0101011
0111111
1000000
1010000
1100000
1111101

B: Base
L: Local
O: Other
M: Merged flags from mergestate.resolve(), i.e. what's called "local"

when we get to `simplemerge.simplemerge()`

C: commonflags in simplemerge.simplemerge(), i.e. M & O
A: addedflags in simplemerge.simplemerge(), i.e. (M ^ O) - B
R: Re-merged flags simplemerge.simplemerge(), i.e. C | A

As you can see, the re-merged flags are always unchanged compared to
the initial merged flags (R equals M).

Therefore, this patch effectively backs out 84614212ae39. (I might
later refactor this code to have the flags explicitly passed in.)

simplemerge.simplemerge() is also called from
contrib/simplemerge.py, but that code never passes any flags.

Differential Revision: https://phab.mercurial-scm.org/D11879