git-prompt: include sparsity state as well

git-prompt: include sparsity state as well

git-prompt includes the current branch, a bunch of single character
mini-state displayers, and some much longer in-progress state
notifications.  The current branch is always shown.  The single
character mini-state displayers are all off by default (they are not
self explanatory) but each has an environment variable for turning it
on.  The in-progress state notifications provide no configuration
options for turning them off, and can be up to 15 characters long (e.g.
"|REBASE (12/18)" or "|CHERRY-PICKING").

The single character mini-state tends to be used for things like "Do you
have any stashes in refs/stash?" or "Are you ahead or behind of
upstream?".  These are things which users can take advantage of but do
not affect most normal git operations.  The in-progress states, by
contrast, suggest the user needs to interact differently and may also
prevent some normal operations from succeeding (e.g. git switch may show
an error instead of switching branches).

Sparsity is like the in-progress states in that it suggests a
fundamental different interaction with the repository (many of the files
from the repository are not present in your working copy!).  A few
commits ago added sparsity information to wt_longstatus_print_state(),
grouping it with other in-progress state displays.  We do similarly here
with the prompt and show the extra state, by default, with an extra
    |SPARSE
This state can be present simultaneously with the in-progress states, in
which case it will appear before the other states; for example,
    (branchname|SPARSE|REBASE 6/10)

The reason for showing the "|SPARSE" substring before other states is to
emphasize those other states.  Sparsity is probably not going to change
much within a repository, while temporary operations will.  So we want
the state changes related to temporary operations to be listed last, to
make them appear closer to where the user types and make them more
likely to be noticed.

The fact that sparsity isn't just cached metadata or additional
information is what leads us to show it more similarly to the
in-progress states, but the fact that sparsity is not transient like the
in-progress states might cause some users to want an abbreviated
notification of sparsity state or perhaps even be able to turn it off.
Allow GIT_PS1_COMPRESSSPARSESTATE to be set to request that it be
shortened to a single character ('?'), and GIT_PS1_OMITSPARSESTATE to be
set to request that sparsity state be omitted from the prompt entirely.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>