Git's gitk needs TK's wish to be found in PATH (#8360)

* Git's gitk needs TK's wish to be found in PATH

* Add a _runtime_ dependency on `tk`
* Add an environment rule to add the path to TK's `wish` program to $PATH for
  the generated `git` modulefile.

* Make TclTk run environment an optional variant for git.

* Cleanup based on PR recommendations.

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

@@ -160,6 +160,9 @@ class Git(AutotoolsPackage):
             placement='git-manpages',
             when='@{0}'.format(release['version']))
 
+    variant('tcltk', default=False,
+            description='Gitk: provide Tcl/Tk in the run environment')
+
     depends_on('curl')
     depends_on('expat')
     depends_on('gettext')
@@ -174,6 +177,7 @@ class Git(AutotoolsPackage):
     depends_on('automake', type='build')
     depends_on('libtool',  type='build')
     depends_on('m4',       type='build')
+    depends_on('tk',       type=('build', 'link'), when='+tcltk')
 
     # See the comment in setup_environment re EXTLIBS.
     def patch(self):
@@ -202,7 +206,7 @@ def setup_environment(self, spack_env, run_env):
     def configure_args(self):
         spec = self.spec
 
-        return [
+        configure_args = [
             '--with-curl={0}'.format(spec['curl'].prefix),
             '--with-expat={0}'.format(spec['expat'].prefix),
             '--with-iconv={0}'.format(spec['libiconv'].prefix),
@@ -212,6 +216,14 @@ def configure_args(self):
             '--with-zlib={0}'.format(spec['zlib'].prefix),
         ]
 
+        if '+tcltk' in self.spec:
+            configure_args.append('--with-tcltk={0}'.format(
+                self.spec['tk'].prefix.bin.wish))
+        else:
+            configure_args.append('--without-tcltk')
+
+        return configure_args
+
     @run_after('configure')
     def filter_rt(self):
         if sys.platform == 'darwin':