extdiff: add comments and minor variable renames diffpatch()
Some variable names were now confusing as we refactored the code in a separate
function. For example, `node1a` leads to ideas why `1a` and not `1`. The
variable storing path to patch file was named as `dirX` instead of `fileX`.
Renamed these variables and added couple of comments.
Differential Revision: https://phab.mercurial-scm.org/D8689
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