bash completion: do not expand tilde

[maxnikulin]

Make behavior of bash completion for ag consistent with ls, man, etc. in respect to tilde expansion to home directory path. Do not replace ~ by /home/user in response to [Tab] in the following cases:

ag pattern ~us
ag pattern ~/src

Completion of file names is still handled by _filedir. As to other utilities, e.g. ls uses _longopt function that does not call
_expand at all, while man has call of _expand but it is bypassed for file names.

Expansion of ~ is controlled by the following setting in inputrc:

set expand-tilde on

For details see info "(bash) Readline Init File Syntax". Alternatively M-& (tilde-expand command) may be used to expand tilde at particular point info "(bash) Miscellaneous Commands".

If you are going to revert this commit, please, add a comment with an example when _filedir is not enough and _expand is really necessary.

[maxnikulin]

Feel free to decline this pull request if I missed some use case.

My story is the following. I do not like when ~ is expanded by /home/user since it makes commands longer and more noisy. For a decade I had a hack from [1] redefining _expand in my .bashrc file. Recently I decided to try if the problem has been fixed and the snippet may be removed. For some period of time I was rather happy since everything worked as I prefer, but I have discovered that for ag tilde is still expanded. I have dug deeper and have followed links to other Ubuntu and Debian bugs mentioned in [1]. Some patch was applied and reverted [2]. I do not find the argument related to scp convincing, from my point of view it is scp completion that should be fixed in such case. On the other hand, since behavior of most commands is consistent with my expectations, I have no reason to complain that implementation of _expand should be patched. As a result I think that bash completion for ag needs a fix.

[1] lp:622403 bash-completion: readline expand-tilde not acknowledged
[2] Ville Skyttä [Bash-completion-devel] _expand, scp and quoting