Resolve HEAD in ksurl to actual hash

When the image build configuration specifies kickstart URL as a HEAD of
a git repo, pungi now figures out what the actual hash of that commit is
and uses that hash instead. This might make logs clearer and should
prevent potential problems if someone pushes to that repo during
composing.

Documentation is updated to mention this.

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

doc/configuration.rst

@@ -480,7 +480,8 @@ Image Build Settings
 
 .. note::
     Config can contain anything what is accepted by
-    koji image-build --config configfile.ini
+    ``koji image-build --config configfile.ini``
+
     Repo is currently the only option which is being automatically transformed
     into a string.
 
@@ -488,6 +489,10 @@ Image Build Settings
     The 'format' attr is [('image_type', 'image_suffix'), ...].
     productmd should ideally contain all of image types and suffixes.
 
+    If ``ksurl`` ends with ``#HEAD``, Pungi will figure out the SHA1 hash of
+    current HEAD and use that instead.
+
+
 Example
 -------
 ::
@@ -501,6 +506,7 @@ Example
                     'target': 'koji-target-name',
                     'ksversion': 'F23', # value from pykickstart
                     'version': '23',
+                    # correct SHA1 hash will be put into the URL below automatically
                     'ksurl': 'https://git.fedorahosted.org/git/spin-kickstarts.git?somedirectoryifany#HEAD',
                     'kickstart': "fedora-docker-base.ks",
                     'repo': ["http://someextrarepos.org/repo", "ftp://rekcod.oi/repo].