Merge branch 'cc/skip-to-optional-val'

Introduce a helper to simplify code to parse a common pattern that
expects either "--key" or "--key=<something>".

* cc/skip-to-optional-val:
t4045: reindent to make helpers readable
diff: add tests for --relative without optional prefix value
diff: use skip_to_optional_arg_default() in parsing --relative
diff: use skip_to_optional_arg_default()
diff: use skip_to_optional_arg()
index-pack: use skip_to_optional_arg()
git-compat-util: introduce skip_to_optional_arg()

diff --cc diff.c
Simple merge

diff --cc strbuf.c
Simple merge