Set the mark on the
file you want diff to compare, put
the cursor on the other file, then type =. Emacs compares the two files and opens a
window with a
*diff* buffer containing the output
from the command.
You must use C-SPC to mark the region to use diff, the current point is the new file(if compare with old file...), use C-x C-x to change mark and point
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