gnu: git: Update to 2.18.0.

* gnu/packages/version-control.scm (git): Update to 2.18.0.
[inputs]: Add BASH.
[arguments]: In #:make-flags, define SHELL_PATH and TEST_SHELL_PATH.
Add #:disallowed-references.  Drop /bin/sh substitution from Makefile.  Add
phase to prevent the new BASH from ending up in PATH.

gnu/packages/version-control.scm

@@ -53,6 +53,7 @@ (define-module (gnu packages version-control)
   #:use-module (gnu packages autotools)
   #:use-module (gnu packages documentation)
   #:use-module (gnu packages base)
+  #:use-module (gnu packages bash)
   #:use-module (gnu packages bison)
   #:use-module (gnu packages boost)
   #:use-module (gnu packages check)
@@ -144,14 +145,14 @@ (define-public git
    (name "git")
    ;; XXX When updating Git, check if the special 'git:src' input to cgit needs
    ;; to be updated as well.
-   (version "2.17.1")
+   (version "2.18.0")
    (source (origin
             (method url-fetch)
             (uri (string-append "mirror://kernel.org/software/scm/git/git-"
                                 version ".tar.xz"))
             (sha256
              (base32
-              "0pm6bdnrrm165k3krnazxcxadifk2gqi30awlbcf9fism1x6w4vr"))))
+              "14hfwfkrci829a9316hnvkglnqqw1p03cw9k56p4fcb078wbwh4b"))))
    (build-system gnu-build-system)
    (native-inputs
     `(("native-perl" ,perl)
@@ -164,7 +165,7 @@ (define-public git
                 version ".tar.xz"))
           (sha256
            (base32
-            "0m7grrwsqaihdgcgaicxiy4rlqjpa75n5wl6hi2qhi33xa34gmc3"))))))
+            "15k04s9pcc5wkmlfa8x99nbgczjbx0c91767ciqmjy9kwsavxqws"))))))
    (inputs
     `(("curl" ,curl)
       ("expat" ,expat)
@@ -173,6 +174,10 @@ (define-public git
       ("python" ,python-2) ; CAVEAT: incompatible with python-3 according to INSTALL
       ("zlib" ,zlib)
 
+      ;; Note: we keep this in inputs rather than native-inputs to work around
+      ;; a problem in 'patch-shebangs'; see <https://bugs.gnu.org/31952>.
+      ("bash-for-tests" ,bash)
+
       ;; For 'gitweb.cgi'
       ("perl-cgi" ,perl-cgi)
 
@@ -196,10 +201,23 @@ (define-public git
    (arguments
     `(#:make-flags `("V=1"                        ;more verbose compilation
 
+                     ,(string-append "SHELL_PATH="
+                                     (assoc-ref %build-inputs "bash")
+                                     "/bin/sh")
+
+                     ;; Tests require a bash with completion support.
+                     ,(string-append "TEST_SHELL_PATH="
+                                     (assoc-ref %build-inputs "bash-for-tests")
+                                     "/bin/bash")
+
                      ;; By default 'make install' creates hard links for
                      ;; things in 'libexec/git-core', which leads to huge
                      ;; nars; see <https://bugs.gnu.org/21949>.
                      "NO_INSTALL_HARDLINKS=indeed")
+
+      ;; Make sure the full bash does not end up in the final closure.
+      #:disallowed-references (,bash)
+
       #:test-target "test"
 
       ;; Tests fail randomly when parallel: <https://bugs.gnu.org/29512>.
@@ -212,13 +230,23 @@ (define-public git
                                              "/bin/wish8.6")) ; XXX
 
       #:modules ((srfi srfi-1)
+                 (srfi srfi-26)
                  ,@%gnu-build-system-modules)
       #:phases
       (modify-phases %standard-phases
+        (add-after 'unpack 'modify-PATH
+          (lambda* (#:key inputs #:allow-other-keys)
+            (let ((path (string-split (getenv "PATH") #\:))
+                  (bash-full (assoc-ref inputs "bash-for-tests")))
+              ;; Drop the test bash from PATH so that (which "sh") and
+              ;; similar does the right thing.
+              (setenv "PATH" (string-join
+                              (remove (cut string-prefix? bash-full <>) path)
+                              ":"))
+              #t)))
         (add-after 'configure 'patch-makefiles
           (lambda _
             (substitute* "Makefile"
-              (("/bin/sh") (which "sh"))
               (("/usr/bin/perl") (which "perl"))
               (("/usr/bin/python") (which "python")))
             #t))