Merge doc: set DAIKONDIR when cloning; explain the .orig files of patch -b

[mernst]

Two small fixes to doc/valgrind-merge.texinfo:

• "Obtain a copy of Daikon and Fjalar" sets DAIKONDIR right after setting INV. "Getting Started" sets DAIKONDIR=$INV/daikon, but a user who follows the cloning example sets INV only afterward.
• "Merging the binutils changes" presents the two ways to keep copies of dwarf.c and readelf.c from before the patch, for the visual merge tool: copying them by hand, or adding -b to the actual (not the dry-run) patch command that applies binutils.patch. It explains what patch -b does: it saves the original version of each file that it modifies in a .orig file (without -b, patch saves one only when the patch does not match a file exactly), which is useful for reviewing and undoing the changes and for the visual merge tool. It warns that a second run of patch -b overwrites each .orig file with the already-patched file. It says to remove the .orig files after the merge and how to find them, because .gitignore hides them from git status.

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