Fix index blob for merges with CRLF translations

Fix index blob for merges with CRLF translations

Commit fc7d407 corrected line endings for working tree files resulting
from merges when CRLF translations are to be done. However, that also
resulted in the file content being put as-is into the index, which is
wrong. The index must contain the file content with reverse CRLF
translations applied.

With core.autocrlf=true, the working tree file should have CR-LF, but
the index blob must still contain only LF.

Fix this oversight and apply the inverse translation when updating the
index, similar to what is done in AddCommand.

Bug: 499615
Change-Id: I3a33931318bdb580b2390f3450f91ea8f258a6a4
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>