Add bash completions for `git prune`

[johnlinp]

Currently, typing git pr<TAB> won't auto complete to git prune. We should add a bash completion script for it.

[Denton-L]

This is intentional. git prune is considered a "plumbing" command so it shouldn't be exposed to end-users directly. The "porcelain" command is git gc.

Taken from the manpages for git prune,

In most cases, users will not need to call git prune directly, but should instead call git gc, which handles pruning along with many other housekeeping tasks.

[johnlinp]

Sometimes, when I run git status, I would see messages like:

warning: There are too many unreachable loose objects; run 'git prune' to remove them.

Therefore, I would assume that the command git prune is exposed to end-users.

[Denton-L]

Interesting... So from what I can tell, that warning comes from git gc, introduced in a087cc9 (git-gc --auto: protect ourselves from accumulated cruft, 2007-09-17). At that time, git gc --auto did not prune by default although that has changed from then to now. That raises two questions for me:

1. Why would git status emit this message at all? I don't see any codepath where it calls run_auto_gc().

2. Do you have gc.pruneExpire set or something? Or do you have a workflow that creates a lot of discarded objects? Otherwise, whenever gc runs, it should automatically prune unless you're using an ancient version of Git.

Anyway, there's probably one actionable change that we should do which is change that message in builtin/gc.c to read "run 'git gc --auto'" instead.

[johnlinp]

1. Maybe I recalled incorrectly... Perhaps it's from git push or git pull...
2. I don't have gc.pruneExpire set, and I'm not using some ancient version off Git. I don't think I am using some special workflow, either.

I will submit a PR to change the message. Thank you for the detailed analysis.

[johnlinp]

With the discussion in #651, I will submit another PR for git gc --prune=now.

[johnlinp]

As the discussion in #652, I will start working on providing bash completions for git prune.

[johnlinp]

#667 was merged into master. Closing this issue.