merge: store cases when a file is absent post merge in commitinfo

Some merges can result in file being absent form working directory. This can be
one of file was kept deleted or file was removed by merge code.

User might revert the file back before committing. In such cases we will like to
have better handling and create new filenodes.

We store this info in mergestate as commitinfo so that we can use it while
committing to create new filenode if required.

Differential Revision: https://phab.mercurial-scm.org/D9003
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