Add documentation for git-map-branches, git-map, git-nav-downstream,

docs/src/git-map-branches.txt

Index: docs/src/git-map-branches.txt
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+git-map-branches(1)
+===================
+
+NAME
+----
+git-map-branches -
+include::_git-map-branches_desc.helper.txt[]
+
+SYNOPSIS
+--------
+[verse]
+'git map-branches'
+
+DESCRIPTION
+-----------
+
+Git map-branches displays all local branches such that:
+
+* Current branch is [aqua]#cyan#.
+** The branch which will be modified with git-commit is denoted with an asterisk
+	 (`*`) after the name.
+* Local branches are [green]#green#.
+* Remote branches are [red]#red# (usually, the root of all other branches).
+* `{NO UPSTREAM}` is a special placeholder in [fuchsia]#magenta#.
+** Branches which have this as their parent are usually misconfigured, and
+	 should be assigned a parent by checking out the branch and running git branch
+	 --set-upstream-to=<correct parent branch>.
+
+NOTE: If multiple branches are on the same commit, they will all be cyan.
+
+EXAMPLE
+-------
+
+Given the hypothetical demo repo in linkgit:git-map[1]\'s EXAMPLE section, and
+assuming that the `frozen_changes` branch was currently checked out, running
+'git map-branches' would result in an output like:
+
+[subs="quotes"]
+----
+[white]**$ git map-branches**
+[red]#origin/master#
+  [green]#cool_feature#
+    [green]#subfeature#
+  [aqua]#frozen_changes *#
+  [green]#master#
+[fuchsia]#{NO UPSTREAM}#
+  [green]#bogus_noparent#
+  [aqua]#duplicate_cool_feature_no_upstream#
+----
+
+SEE ALSO
+--------
+linkgit:git-map[1]
+
+include::_footer.txt[]
+
+// vim: ft=asciidoc noexpandtab: