+--exclude-per-directory=<gitignore>::
+ When running the command with `-u` and `-m` options, the
+ merge result may need to overwrite paths that are not
+ tracked in the current branch. The command usually
+ refuses to proceed with the merge to avoid losing such a
+ path. However this safety valve sometimes gets in the
+ way. For example, it often happens that the other
+ branch added a file that used to be a generated file in
+ your branch, and the safety valve triggers when you try
+ to switch to that branch after you ran `make` but before
+ running `make clean` to remove the generated file. This
+ option tells the command to read per-directory exclude
+ file (usually '.gitignore') and allows such an untracked
+ but explicitly ignored file to be overwritten.