Update git to @2.14.1, requires pcre+jit variant (#5430)

Update the git package to git@2.14.1.

This requires a pcre that has been built with `--enable-jit`, so this
adds a variant to pcre to support that and arranges so that git versions
before 2.14 depend on pcre and git 2.14 and after depend on pcre+jit.

var/spack/repos/builtin/packages/pcre/package.py

@@ -39,6 +39,9 @@ class Pcre(AutotoolsPackage):
 
     patch('intel.patch', when='@8.38')
 
+    variant('jit', default=False,
+            description='Enable JIT support.')
+
     variant('utf', default=True,
             description='Enable support for UTF-8/16/32, '
             'incompatible with EBCDIC.')
@@ -46,6 +49,9 @@ class Pcre(AutotoolsPackage):
     def configure_args(self):
         args = []
 
+        if '+jit' in self.spec:
+            args.append('--enable-jit')
+
         if '+utf' in self.spec:
             args.append('--enable-utf')
             args.append('--enable-unicode-properties')