Merge branch 'fc/mergetool-prompt'

Merge branch 'fc/mergetool-prompt'

mergetool.prompt used to default to 'true', always causing a confirmation
"do you really want to run the tool on this path" to be shown.

Among the two purposes the prompt serves, ignore the use case to
confirm that the user wants to view particular path with the named
tool, and make the prompt only to confirm the choice of the tool
made by autodetection and defaulting.  For those who configured the
tool explicitly, the prompt shown for the latter purpose is simply
annoying.

Strictly speaking, this is a backward incompatible change and the
users need to explicitly set the variable to 'true' if they want to
resurrect the now-ignored use case.

* fc/mergetool-prompt:
  mergetool: document the default for --[no-]prompt
  mergetool: run prompt only if guessed tool