[NFC] Improve doc string to mention that paths in diff are used as-is

Summary:
Add --relative to the suggested git-diff one liner. If the user does not
pass this argument, then git will produce a diff with the path relative
to the repository root. If the user's working directory is not the
repository root, then clang-format will complain that the file is not
found. The --relative argument makes git produce a diff with the files
relative to the working directory.

Add note to doc string to warn users about the fact that filenames
embedded in the diff are used as-is with no attempts to "do what they
mean, not what they say"

Reviewers: djasper, alexfh, efriedma, klimek, thakis

Reviewed By: efriedma

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79054

clang/tools/clang-format/clang-format-diff.py

@@ -13,9 +13,13 @@ This script reads input from a unified diff and reformats all the changed
 lines. This is useful to reformat all the lines touched by a specific patch.
 Example usage for git/svn users:
 
-  git diff -U0 --no-color HEAD^ | clang-format-diff.py -p1 -i
+  git diff -U0 --no-color --relative HEAD^ | clang-format-diff.py -p1 -i
   svn diff --diff-cmd=diff -x-U0 | clang-format-diff.py -i
 
+It should be noted that the filename contained in the diff is used unmodified
+to determine the source file to update. Users calling this script directly
+should be careful to ensure that the path in the diff is correct relative to the
+current working directory.
 """
 from __future__ import absolute_import, division, print_function