[BUG] Data loss when committing a new file which is a target of multiple merges

Steps to reproduce:

1. Merge content into a WC in a manner which creates a new file.
2. Before committing, perform a subsequent merge with an adjacent
merge range (e.g. 2:5, then 5:7).
3. Commit the new contents of your WC.

Observed behavior:

After step #2, the file in your WC will contain the expected content.
However, after step #3 the repository will not contain the result of
the second merge. Instead, it only contains the the result through
the end of your first merge (e.g. the "copied from" revision will be
r5, even if the second operation merged in more changes).

Expected behavior:

The repository and WC should contain the same data. The "copied from"
info should be updated to account for the end revision of the first
contiguous merge range applied to your WC (across multiple merges).

I've attached a reproduction recipe and output. In the recipe, the
"copied from" info should cite r6 of foo.txt. Instead, it cites r5
(the end of the first merge).