Add support for git-credential-helper

This patch adds an additional field `options` to scm_dict, which can be
used to provide additional information to the backends.

It implements a single new option for GitWrapper. This option allows
setting a custom git credentials wrapper. This can be useful if Pungi
needs to get files from a git repository that requires authentication.

The helper can be as simple as this (assuming the username is already
provided in the url):

    #!/bin/sh
    echo password=i-am-secret

The helper would need to be referenced by an absolute path from the
pungi configuration, or prefixed with ! to have git interpret it as a
shell script and look it up in PATH.

See https://git-scm.com/docs/gitcredentials for more details.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>
JIRA: RHELCMP-11808

doc/scm_support.rst

@@ -41,6 +41,14 @@ which can contain following keys.
 * ``command`` -- defines a shell command to run after Git clone to generate the
   needed file (for example to run ``make``). Only supported in Git backend.
 
+* ``options`` -- a dictionary of additional configuration options. These are
+  specific to different backends.
+
+  Currently supported values for Git:
+
+  * ``credential_helper`` -- path to a credential helper used to supply
+    username/password for remotes that require authentication.
+
 
 Koji examples
 -------------