Support for Submodule configuration submodule.<name>.ignore

For each submodule native git allows to configure which modifications to
submodules should be ignored by the status command. It is possible to
ignore "none", "all", "dirty", "untracked" [1]. This configuration is
now supported by IndexDiff. The StatusCommand offers the possibility to
specify this mode.

[1] http://git-scm.com/docs/gitmodules

Change-Id: Ifd81d574a680f9b4152945ba70f8ec4af4f452c9
RebaseCommand should ignore submodule modifications

Before a rebase happens the RebaseCommand checks that the working tree
is clean. We don't want to start a rebase on a dirty working tree. If
the working tree is dirty a rebase should not be allowed. But
RebaseCommand should ignore modifications done to submodules. E.g. if a
submodules HEAD points to <x> but the root repository has in index that
the submodule should point to <y> then this should not prohibit a
rebase. Also native git allows a rebase in this case. Since jgit's
StatusCommand has learned to ignore submodule changes this is now used
by the RebaseCommand to determine the repository state correctly.

Bug: 446922
Change-Id: I487bf7484dca3f5501e6e514584e2871524eea19
Enhance SubmoduleWalk with a fast check whether a repo contains
submodules

Change-Id: Id37efb4f4dd77f3b8eb5607d15d32adeda3992d4