style: Add git-source rule.

* guix/scripts/style.scm (transform-to-git-fetch)
(url-fetch->git-fetch): New procedures.
* doc/guix.texi: Add entry for git-source.
* tests/style.scm: Add tests for url-fetch->git-fetch.

Change-Id: I6192fba4d84619b81fbc75850542b9dbd2326d4a
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -15801,6 +15801,49 @@ Note that changes made by the @code{arguments} rule do not entail a
 rebuild of the affected packages.  Furthermore, if a package definition
 happens to be using G-expressions already, @command{guix style} leaves
 it unchanged.
+
+@item git-source
+If the @code{home-page} is a Git repository (as per
+@code{git-repository-url?}), and the actual Git repository is tagged
+with @code{version} or @code{(string-append ``v'' version)}, change the
+package origin to the @code{git-fetch} method
+(@pxref{origin Reference}).  Consider this example:
+
+@lisp
+(define-public guile-json-4
+  (package
+    (inherit guile-json-3)
+    (name "guile-json")
+    (version "4.7.3")
+    (source (origin
+              (method url-fetch)
+              (uri (string-append "mirror://savannah/guile-json/guile-json-"
+                                  version ".tar.gz"))
+              (sha256
+               (base32
+                "127k2xc07w1gnyqs40z4865l8p3ra5xgpcn569dz04lxsa709fiq"))))))
+@end lisp
+
+@noindent
+Running @command{guix style -S git-source} on this package would rewrite
+its @code{source} field like to:
+
+@lisp
+(define-public guile-json-4
+  (package
+    (inherit guile-json-3)
+    (name "guile-json")
+    (version "4.7.3")
+    (source (origin
+              (method git-fetch)
+              (uri (git-reference (url
+                                   "https://github.com/aconchillo/guile-json")
+                                  (commit version)))
+              (file-name (git-file-name name version))
+              (sha256 (base32
+                       "0akhm8xjv8fl55fyq0w6c9c6hi5j7mifjx01w07np7qg1cjl9f06"))))))
+@end lisp
+
 @end table
 
 @item --list-stylings