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

Documentation/git-mergetool.txt

@@ -71,11 +71,13 @@ success of the resolution after the custom tool has exited.
 --no-prompt::
 	Don't prompt before each invocation of the merge resolution
 	program.
+	This is the default if the merge resolution program is
+	explicitly specified with the `--tool` option or with the
+	`merge.tool` configuration variable.
 
 --prompt::
-	Prompt before each invocation of the merge resolution program.
-	This is the default behaviour; the option is provided to
-	override any configuration settings.
+	Prompt before each invocation of the merge resolution program
+	to give the user a chance to skip the path.
 
 TEMPORARY FILES
 ---------------

git-mergetool.sh

@@ -277,7 +277,7 @@ merge_file () {
 	echo "Normal merge conflict for '$MERGED':"
 	describe_file "$local_mode" "local" "$LOCAL"
 	describe_file "$remote_mode" "remote" "$REMOTE"
-	if "$prompt" = true
+	if test "$guessed_merge_tool" = true || test "$prompt" = true
 	then
 		printf "Hit return to start merge resolution tool (%s): " "$merge_tool"
 		read ans || return 1
@@ -315,7 +315,8 @@ merge_file () {
 	return 0
 }
 
-prompt=$(git config --bool mergetool.prompt || echo true)
+prompt=$(git config --bool mergetool.prompt)
+guessed_merge_tool=false
 
 while test $# != 0
 do
@@ -373,7 +374,14 @@ prompt_after_failed_merge () {
 
 if test -z "$merge_tool"
 then
-	merge_tool=$(get_merge_tool "$merge_tool") || exit
+	# Check if a merge tool has been configured
+	merge_tool=$(get_configured_merge_tool)
+	# Try to guess an appropriate merge tool if no tool has been set.
+	if test -z "$merge_tool"
+	then
+		merge_tool=$(guess_merge_tool) || exit
+		guessed_merge_tool=true
+	fi
 fi
 merge_keep_backup="$(git config --bool mergetool.keepBackup || echo true)"
 merge_keep_temporaries="$(git config --bool mergetool.keepTemporaries || echo false)"