fix(make): show unique targets in subdirs

When many targets in subdirectories are defined in Makefile, we
typically want to list the files or subdirectories in the current
directory.  However, when the file in the subdirectory is unique, we
may generate the full path.  See also the discussion on GitHub:

scop#544
scop#546 (comment)

completions/make

@@ -46,7 +46,7 @@ _make_target_extract_script()
     /^$/ {                                      # end of target block
       x;                                        # unhold target
       /^$/d;                                    # dont print blanks
-      s|^${dirname_re-}\(.\{${#basename}\}[^:/]*/\{0,1\}\)[^:]*:.*$|${output}|p;
+      s|^${dirname_re-}\(.\{${#basename}\}[^:]*\):.*$|${output}|p;
       d;                                        # hide any bugs
     }
 
@@ -87,6 +87,56 @@ EOF
 EOF
 }
 
+# Truncate the non-unique filepaths in COMPRELY to only generate unique
+# directories or files.  This function discards the files under subdirectories
+# unless the path is unique under each subdirectory and instead generate the
+# subdirectory path.  For example, when there are two candidates, "abc/def" and
+# "abc/xyz", we generate "abc/" instead of generating both candidates directly.
+# When there is only one candidate "abc/def", we generate the full path
+# "abc/def".
+#
+# @var[in] cur
+# @var[in] mode
+# @var[in,out] COMPREPLY
+_comp_make__truncate_non_unique_paths()
+{
+    local prefix=$cur
+    [[ $mode == -d ]] && prefix=
+    if ((${#COMPREPLY[@]} > 0)); then
+        # collect the possible completions including the directory names in
+        # `paths' and count the number of children of each subdirectory in
+        # `nchild'.
+        local -A paths nchild
+        local target
+        for target in "${COMPREPLY[@]}"; do
+            local path=${target%/}
+            while [[ ! ${paths[$path]+set} ]] &&
+                paths[$path]=1 &&
+                [[ $path == "$prefix"*/* ]]; do
+                path=${path%/*}
+                nchild[$path]=$((${nchild[$path]-0} + 1))
+            done
+        done
+
+        COMPREPLY=()
+        local nreply=0
+        for target in "${!paths[@]}"; do
+            # generate only the paths that do not have a unique child and whose
+            # all parent and ancestor directories have a unique child.
+            ((${nchild[$target]-0} == 1)) && continue
+            local path=$target
+            while [[ $path == "$prefix"*/* ]]; do
+                path=${path%/*}
+                ((${nchild[$path]-0} == 1)) || continue 2
+            done
+
+            # suffix `/' when the target path is a subdiretory, which has
+            # at least one child.
+            COMPREPLY[nreply++]=$target${nchild[$target]+/}
+        done
+    fi
+}
+
 _make()
 {
     local cur prev words cword split comp_args
@@ -172,6 +222,8 @@ _make()
             ${makef+"${makef[@]}"} "${makef_dir[@]}" .DEFAULT 2>/dev/null |
             command sed -ne "$script"))
 
+        _comp_make__truncate_non_unique_paths
+
         if [[ $mode != -d ]]; then
             # Completion will occur if there is only one suggestion
             # so set options for completion based on the first one