git-prompt: make long upstream state indicator consistent

git-prompt: make long upstream state indicator consistent

Use a pipe as a separator before long upstream state indicator. This is
consistent with long state indicators for sparse and in-progress
operations (e.g. merge).

For comparison, `__git_ps1` examples without upstream state indicator:
(main)
(main %)
(main *%)
(main|SPARSE)
(main %|SPARSE)
(main *%|SPARSE)
(main|SPARSE|REBASE 1/2)
(main %|SPARSE|REBASE 1/2)

Note that if there are long state indicators, they appear after short
state indicators if there are any, or after the branch name if there are
no short state indicators. Each long state indicator begins with a pipe
(`|`) as a separator.

Before/after examples with long upstream state indicator:
| Before                          | After                           |
| ------------------------------- | ------------------------------- |
| (main u=)                       | (main|u=)                       |
| (main u= origin/main)           | (main|u= origin/main)           |
| (main u+1)                      | (main|u+1)                      |
| (main u+1 origin/main)          | (main|u+1 origin/main)          |
| (main % u=)                     | (main %|u=)                     |
| (main % u= origin/main)         | (main %|u= origin/main)         |
| (main % u+1)                    | (main %|u+1)                    |
| (main % u+1 origin/main)        | (main %|u+1 origin/main)        |
| (main|SPARSE u=)                | (main|SPARSE|u=)                |
| (main|SPARSE u= origin/main)    | (main|SPARSE|u= origin/main)    |
| (main|SPARSE u+1)               | (main|SPARSE|u+1)               |
| (main|SPARSE u+1 origin/main)   | (main|SPARSE|u+1 origin/main)   |
| (main %|SPARSE u=)              | (main %|SPARSE|u=)              |
| (main %|SPARSE u= origin/main)  | (main %|SPARSE|u= origin/main)  |
| (main %|SPARSE u+1)             | (main %|SPARSE|u+1)             |
| (main %|SPARSE u+1 origin/main) | (main %|SPARSE|u+1 origin/main) |

Signed-off-by: Justin Donnelly <justinrdonnelly@gmail.com>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>