Fix issue #338 - Fixes behaviour if `/tmp` dir does not exist

[mskalski]

Private index was tried to create in non-existent location, no error was ever displayed, and then unconditionally GIT_INDEX_FILE variable was set pointing to non-existent path. Git used this location and reported many changed files.

Additionally in case of non-existent index file most of files were marked with D in second loop pass of gitstatus.sh which was wrongly identified as 'Stashed'.

It is common on Android devices - there is no /tmp directory Termux creates one and sets TMPDIR variable, which is commonly honoured by most of unix utilities.

[guenhter]

@mskalski Could you split the two changes into two separate commits, then I am happy to merge that one.

[mskalski]

@mskalski Could you split the two changes into two separate commits, then I am happy to merge that one.

Sure, done (rebased on current master).

Could you also look at my third PR #458, with fixes for git_prompt_help and git_prompt_examples?

[guenhter]

@mskalski could you please again change your commits so that the title of the commit is a statement what the commit contains instead of the issue-text.

[guenhter]

@mskalski sorry for being so picky. Find this now merged. Thx for your contribution

[etpinard]

Thanks very much for this PR. Works great in my termux instance!