alternates: accept double-quoted paths

We read lists of alternates from objects/info/alternates
files (delimited by newline), as well as from the
GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable
(delimited by colon or semi-colon, depending on the
platform).

There's no mechanism for quoting the delimiters, so it's
impossible to specify an alternate path that contains a
colon in the environment, or one that contains a newline in
a file. We've lived with that restriction for ages because
both alternates and filenames with colons are relatively
rare, and it's only a problem when the two meet. But since
722ff7f87 (receive-pack: quarantine objects until
pre-receive accepts, 2016-10-03), which builds on the
alternates system, every push causes the receiver to set
GIT_ALTERNATE_OBJECT_DIRECTORIES internally.

It would be convenient to have some way to quote the
delimiter so that we can represent arbitrary paths.

The simplest thing would be an escape character before a
quoted delimiter (e.g., "\:" as a literal colon). But that
creates a backwards compatibility problem: any path which
uses that escape character is now broken, and we've just
shifted the problem. We could choose an unlikely escape
character (e.g., something from the non-printable ASCII
range), but that's awkward to use.

Instead, let's treat names as unquoted unless they begin
with a double-quote, in which case they are interpreted via
our usual C-stylke quoting rules. This also breaks
backwards-compatibility, but in a smaller way: it only
matters if your file has a double-quote as the very _first_
character in the path (whereas an escape character is a
problem anywhere in the path).  It's also consistent with
many other parts of git, which accept either a bare pathname
or a double-quoted one, and the sender can choose to quote
or not as required.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -859,6 +859,12 @@ Git so take care if using a foreign front-end.
 	specifies a ":" separated (on Windows ";" separated) list
 	of Git object directories which can be used to search for Git
 	objects. New objects will not be written to these directories.
++
+	Entries that begin with `"` (double-quote) will be interpreted
+	as C-style quoted paths, removing leading and trailing
+	double-quotes and respecting backslash escapes. E.g., the value
+	`"path-with-\"-and-:-in-it":vanilla-path` has two paths:
+	`path-with-"-and-:-in-it` and `vanilla-path`.
 
 `GIT_DIR`::
 	If the `GIT_DIR` environment variable is set then it