filemerge: show warning about choice of :prompt only at an actual fallback
Before this patch, internal merge tool :prompt shows "no tool found to
merge FILE" line, even if :prompt is explicitly specified as a tool to
be used.
This patch shows warning message about choice of :prompt only at an
actual fallback, in which case any tool is rejected by capability for
binary or symlink.
It is for backward compatibility to omit warning message in
"changedelete" case.
Welcome to Mercurial for Windows!
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