Warn if git is missing

[lazka]

Using the bash script in docker fails to get the config because by default most docker images don't have git installed.

I figured it out eventually, but a warning/error would have been nice.

[stevepeak]

@lazka We have another technique where you export the report(s) out of docker and upload them with git as a command. See here: https://docs.codecov.io/docs/testing-with-docker

[Midnighter]

Still, on that very page a warning that the technique inside requires git would have saved us quite a bit of trouble.

[rgov]

I was going to file the same bug for curl. Is there a list of all the tools that the script relies on that are not included in, say, a base Debian image? Can these all be tested for by the script?

[thomasrockhu]

Fixed via #394