ResolveMerger: choose OURS on gitlink when ignoreConflicts

ResolveMerger: choose OURS on gitlink when ignoreConflicts

Option ignoreConflicts is used when a caller want to create a virtual
commit and use it in a future merge (recursive merge) or show it on
UI (e.g. Gerrit). According to contract in case of ignoreConflicts
ResolveMerger should populate only stage 0 for entries with merge
conflicts as if there is no conflict. Current implementation breaks
this contract for cases when gitlink revision is ambiguous.

Therefore, always select 'ours' when we merge in ignoreConflicts mode.
This will satisfy the contract contract, so recursive merge can
succeed, however it is an arbitrary decision, so it is not guaranteed
to select best GITLINK in all cases.

GITLINK merging is a special case of recursive merge because of
limitations of GITLINK type of entry. It can't contain more than 1 sha-1
so jgit can't write merge conflicts in place like it can with a blob.
Ideally we could signal the conflict with a special value (like
'0000...'), but that must be supported by all tooling (git fsck, c-git)."

Signed-off-by: Demetr Starshov <dstarshov@google.com>
Change-Id: Id4e9bebc8e828f7a1ef9f83259159137df477d89