Merge branch 'jc/optional-path'

Configuration variables that take a pathname as a value
(e.g. blame.ignorerevsfile) can be marked as optional by prefixing
":(optoinal)" before its value.

* jc/optional-path:
  parseopt: values of pathname type can be prefixed with :(optional)
  config: values of pathname type can be prefixed with :(optional)
  t7500: fix GIT_EDITOR shell snippet
  t7500: make each piece more independent

Documentation/config.adoc

@@ -357,7 +357,9 @@ compiled without runtime prefix support, the compiled-in prefix will be
 substituted instead. In the unlikely event that a literal path needs to
 be specified that should _not_ be expanded, it needs to be prefixed by
 `./`, like so: `./%(prefix)/bin`.
-
++
+If prefixed with `:(optional)`, the configuration variable is treated
+as if it does not exist, if the named path does not exist.
 
 Variables
 ~~~~~~~~~

Documentation/gitcli.adoc

@@ -216,6 +216,20 @@ $ git describe --abbrev=10 HEAD  # correct
 $ git describe --abbrev 10 HEAD  # NOT WHAT YOU MEANT
 ----------------------------
 
+
+Magic filename options
+~~~~~~~~~~~~~~~~~~~~~~
+Options that take a filename allow a prefix `:(optional)`. For example:
+
+----------------------------
+git commit -F :(optional)COMMIT_EDITMSG
+# if COMMIT_EDITMSG does not exist, equivalent to
+git commit
+----------------------------
+
+Like with configuration values, if the named file is missing Git behaves as if
+the option was not given at all. See "Values" in linkgit:git-config[1].
+
 NOTES ON FREQUENTLY CONFUSED OPTIONS
 ------------------------------------