Get the deepest common path of any number of directories.

Currently doesn't normalize input paths, so the following fails (although it will match if the repeating slashes are equal):
test "$(path_common /foo/bar //foo//bar//baz; echo x)" = /foo/barx

Successful tests:
test "$(path_common /a/b/c/d /a/b/e/f; echo x)" = /a/bx
test "$(path_common /long/names/foo /long/names/bar; echo x)" = /long/namesx
test "$(path_common / /a/b/c; echo x)" = /x
test "$(path_common a/b/c/d a/b/e/f ; echo x)" = a/bx
test "$(path_common ./a/b/c/d ./a/b/e/f; echo x)" = ./a/bx
test "$(path_common $'\n/\n/\n' $'\n/\n'; echo x)" = $'\n/\n'x
test "$(path_common --/-- --; echo x)" = '--x'
test "$(path_common '' ''; echo x)" = x
test "$(path_common /foo/bar ''; echo x)" = x
test "$(path_common /foo /fo; echo x)" = x
test "$(path_common $'--$`\! *@ \a\b\e\E\f\r\t\v\\\"\' \n' $'--$`\! *@ \a\b\e\E\f\r\t\v\\\"\' \n'; echo x)" = $'--$`\! *@ \a\b\e\E\f\r\t\v\\\"\' \n'x

.bash_aliases

@@ -114,6 +114,29 @@ find_grouped()
     done
 }
 
+path_common()
+{
+    # Get the deepest common path.
+    local common_path="${1:-}"
+    shift # $1 is obviously part of $1
+
+    while true
+    do
+        for path in "$@"
+        do
+            if ! [[ "$path" = "$common_path"* ]]
+            then
+                new_common_path="${common_path%/*}"
+                [ "$new_common_path" = "$common_path" ] && return 1
+                common_path="${new_common_path}"
+                continue 2
+            fi
+        done
+        break
+    done
+    printf %s "$common_path"
+}
+
 # Bash
 bash_timeout()
 {