Limit git commit message to first 10 images

[alex-shpak]

In order to address error mentioned in #1857

ts=2020-06-16T14:06:12.217892013Z caller=loop.go:151 component=sync-loop jobID=2e650640-7405-54e9-1e77-43fa9663e01c state=done success=false err="fork/exec /usr/bin/git: argument list too long"

This PR will change message in case if there are over 10 docker images updated to:

Auto-release multiple (11) images

 - docker.io/image:v1
 - docker.io/image:v10
 - docker.io/image:v11
 - docker.io/image:v2
 - docker.io/image:v3
 - docker.io/image:v4
 - docker.io/image:v5
 - docker.io/image:v6
 - docker.io/image:v7
 - docker.io/image:v8
 ...

In our case issue was that after few failed attempts to commit to git repo there was large amount of automated releases to apply, which resulted in mentioned error.

I hope to merge this fix, it has high impact on our environment.
Thank you!

[alex-shpak]

Hi! @stefanprodan @hiddeco
Can you please take a glance at this PR, it is quite small and this issue causes problems for bigger environments.

[matscederholm]

Serious impact for us, could you please take a look at this?

[hiddeco]

This looks like it would solve the issue to me, but would like @squaremo to have a look too when he is back (next week).

[stefanprodan]

LGTM

[matscederholm]

Thank you!