clang-format.bash: Limit formatting to files selected by options

Now that our one-shot clang-format run is complete we rarely need to
filter the entire tree anymore.  Filter nothing if no options are given
and just print the usage message.  Offer options to specify selection of
some set of files from Git as needed for various workflows.

Utilities/Scripts/clang-format.bash

@@ -17,7 +17,34 @@
 
 usage='usage: clang-format.bash [<options>] [--]
 
-    --clang-format <tool>              Use given clang-format tool.
+    --help                     Print usage plus more detailed help.
+
+    --clang-format <tool>      Use given clang-format tool.
+
+    --amend                    Filter files changed by HEAD.
+    --cached                   Filter files locally staged for commit.
+    --modified                 Filter files locally modified from HEAD.
+    --tracked                  Filter files tracked by Git.
+'
+
+help="$usage"'
+Example to format locally modified files:
+
+    Utilities/Scripts/clang-format.bash --modified
+
+Example to format locally modified files staged for commit:
+
+    Utilities/Scripts/clang-format.bash --cached
+
+Example to format the current topic:
+
+    git filter-branch \
+      --tree-filter "Utilities/Scripts/clang-format.bash --amend" \
+      master..
+
+Example to format all files:
+
+    Utilities/Scripts/clang-format.bash --tracked
 '
 
 die() {
@@ -28,9 +55,15 @@ die() {
 
 # Parse command-line arguments.
 clang_format=''
+mode=''
 while test "$#" != 0; do
     case "$1" in
+    --amend) mode="amend" ;;
+    --cached) mode="cached" ;;
     --clang-format) shift; clang_format="$1" ;;
+    --help) echo "$help"; exit 0 ;;
+    --modified) mode="modified" ;;
+    --tracked) mode="tracked" ;;
     --) shift ; break ;;
     -*) die "$usage" ;;
     *) break ;;
@@ -59,8 +92,18 @@ if ! type -p "$clang_format" >/dev/null; then
     exit 1
 fi
 
+# Select listing mode.
+case "$mode" in
+    '')       echo "$usage"; exit 0 ;;
+    amend)    git_ls='git diff-tree  --diff-filter=AM --name-only HEAD -r --no-commit-id' ;;
+    cached)   git_ls='git diff-index --diff-filter=AM --name-only HEAD --cached' ;;
+    modified) git_ls='git diff-index --diff-filter=AM --name-only HEAD' ;;
+    tracked)  git_ls='git ls-files' ;;
+    *) die "invalid mode: $mode" ;;
+esac
+
 # Filter sources to which our style should apply.
-git ls-files -z -- '*.c' '*.cc' '*.cpp' '*.cxx' '*.h' '*.hh' '*.hpp' '*.hxx' |
+$git_ls -z -- '*.c' '*.cc' '*.cpp' '*.cxx' '*.h' '*.hh' '*.hpp' '*.hxx' |
 
   # Exclude lexer/parser generator input and output.
   egrep -z -v '^Source/cmCommandArgumentLexer\.' |