Allow absolute project dirs and fix AZDO

[lkysow]

Previously, we wouldn't modify the config

    project: /a/b/c

and would use the absolute directory.

Now we prepend ./ and then run Clean() to ensure that every project dir
is relative.

This PR also fixes AZDO to return relative paths from its GetModifiedFiles() function to match the rest of the VCS's.

Fixes #849

[codecov]

Codecov Report

Merging #888 into master will increase coverage by 0.15%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
+ Coverage   71.69%   71.85%   +0.15%     
==========================================
  Files          65       65              
  Lines        5271     5244      -27     
==========================================
- Hits         3779     3768      -11     
+ Misses       1198     1189       -9     
+ Partials      294      287       -7

Impacted Files	Coverage Δ
server/events/vcs/github_client.go	78.81% <ø> (ø)	⬆️
server/events/vcs/bitbucketcloud/client.go	47.32% <ø> (ø)	⬆️
server/events/vcs/gitlab_client.go	39.31% <ø> (ø)	⬆️
server/events/vcs/bitbucketserver/client.go	35.29% <ø> (ø)	⬆️
server/events/yaml/raw/project.go	100% <100%> (ø)	⬆️
server/events/vcs/azuredevops_client.go	73.75% <50%> (+3.96%)	⬆️
server/server.go	63.58% <0%> (+0.58%)	⬆️

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[lkysow]

@mcdafydd can you take a look at this?

[mcdafydd]

Thanks, @lkysow. Looks fine to me.