cgit.c: add 'clone-url' setting with support for macro expansion

The current 'clone-prefix' setting has some known issues:
* All repos get the same 'clone-prefix' value since the setting is not
  adopted during repo registration (in cgitrc, or during scan-path traversal),
  but only when the setting is used.
* The generated clone-urls for a repo is a combination of 'clone-prefix', a
  slash and the repo url. This doesn't work well with e.g. ssh-style urls
  like 'git@example.org:repo.git', since the inserted slash will make the
  repo relative to the filesystem root.
* If 'remove-suffix' is enabled, the generated clone-urls will not work for
  cloning (except for http-urls to cgit itself) since they miss the '.git'
  suffix.

The new 'clone-url' setting is designed to avoid the mentioned issues:
* Each repo adopts the default 'clone-url' when the repo is defined. This
  allows different groups of repos to adopt different values.
* The clone-urls for a repo is generated by expanding environment variables
  in a string template without inserting arbitrary characters, hence any
  kind of clone-url can be generated.
* Macro expansion also eases the 'remove-suffix' pain since it's now
  possible to define e.g. 'clone-url=git://foo.org/$CGIT_REPO_URL.git' for
  a set of repos. A furter improvement would be to define e.g.
  $CGIT_REPO_SUFFIX to '.git' for all repos which had their url prettified,
  or to store the original $CGIT_REPO_URL in e.g. $CGIT_REPO_REAL_URL before
  suffix removal.

Reviewed-by: Ferry Huberts <mailings@hupie.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgitrc.5.txt

@@ -76,6 +76,11 @@ clone-prefix::
 	setting is only used if `repo.clone-url` is unspecified. Default value:
 	none.
 
+clone-url::
+	Space-separated list of clone-url templates. This setting is only
+	used if `repo.clone-url` is unspecified. Default value: none. See
+	also: "MACRO EXPANSION", "FILTER API".
+
 commit-filter::
 	Specifies a command which will be invoked to format commit messages.
 	The command will get the message on its STDIN, and the STDOUT from the
@@ -363,7 +368,7 @@ repo.about-filter::
 
 repo.clone-url::
 	A list of space-separated urls which can be used to clone this repo.
-	Default value: none.
+	Default value: none. See also: "MACRO EXPANSION".
 
 repo.commit-filter::
 	Override the default commit-filter. Default value: none. See also:
@@ -511,6 +516,12 @@ can be accomplished by adding the following line to /etc/cgitrc:
 
 	include=/etc/cgitrc.d/$HTTP_HOST
 
+The following options are expanded during request processing, and support
+the environment variables defined in "FILTER API":
+
+- clone-url
+- repo.clone-url
+
 
 EXAMPLE CGITRC FILE
 -------------------
@@ -520,8 +531,8 @@ EXAMPLE CGITRC FILE
 cache-size=1000
 
 
-# Specify some default clone prefixes
-clone-prefix=git://example.com ssh://example.com/pub/git http://example.com/git
+# Specify some default clone urls using macro expansion
+clone-url=git://foo.org/$CGIT_REPO_URL git@foo.org:$CGIT_REPO_URL
 
 # Specify the css url
 css=/css/cgit.css