fix(cli): limit commit message length when there is too many files

[Panaetius]

closes #3355

With many files, the commit message could get too long for an argument on the command line.

We could save it to a file and pass that to git, but then, what's the point of having an overly long commit message.

[coveralls]

Pull Request Test Coverage Report for Build 4524113903

• 4 of 5 (80.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+85.9%) to 85.934%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
renku/core/util/git.py	4	5	80.0%

Totals
Change from base Build 4524113155:	85.9%
Covered Lines:	25634
Relevant Lines:	29830

---

💛 - Coveralls

[m-alisafaee]

LGTM, thank you!