Add warning when using relative path to app.ini

[guillep2k]

Mitigation for #10082

$ gitea --config ./app.ini doctor
2020/02/01 18:35:38 ...s/setting/setting.go:514:SetCustomPathAndConf() [W] Using 'custom' directory as relative origin for configuration file: '/home/user/custom/app.ini'
2020/02/01 18:35:38 ...s/setting/setting.go:514:SetCustomPathAndConf() [W] Using 'custom' directory as relative origin for configuration file: '/home/user/custom/app.ini'
[ 1 ] Check paths and basic configuration
- Failed to find configuration file at '/home/user/custom/app.ini'.
- If you've never ran Gitea yet, this is normal and '/home/user/custom/app.ini' will be created for you on first run.
- Otherwise check that you are running this command from the correct path and/or provide a `--config` parameter.
Error: can't proceed without a configuration file
$

It beats me why it's showing the warning twice, but I don't think it's too important.

Notice: this warning will be showing in hooks too.

[zeripath]

I think we should just make the breaking change to have it relative to the working directory.

[guillep2k]

I think we should just make the breaking change to have it relative to the working directory.

I'd second that. It's not that big of a deal; it should be an easy fix for any admin if the instance stops working.

[codecov-io]

Codecov Report

Merging #10104 into master will decrease coverage by <.01%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master   #10104      +/-   ##
==========================================
- Coverage   43.49%   43.48%   -0.01%     
==========================================
  Files         567      568       +1     
  Lines       79105    79109       +4     
==========================================
- Hits        34409    34403       -6     
- Misses      40447    40459      +12     
+ Partials     4249     4247       -2

Impacted Files	Coverage Δ
cmd/embedded_stub.go	0% <0%> (ø)
modules/queue/manager.go	60.51% <100%> (-1.54%)	⬇️
services/pull/temp_repo.go	31.62% <0%> (-2.57%)	⬇️
models/unit.go	37.03% <0%> (-2.47%)	⬇️
routers/repo/view.go	39.13% <0%> (-0.87%)	⬇️
models/repo.go	51.46% <0%> (+0.13%)	⬆️
services/pull/patch.go	66.66% <0%> (+0.62%)	⬆️
modules/log/file.go	77.62% <0%> (+2.09%)	⬆️

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