Fix libgit2 test failure with system libgit2 and OpenSSL (JuliaLang#2…

…2722)

The first letter of the error message is in lowercase in some systems,
notably Fedora Rawhide with libgit2 0.26 and OpenSSL 1.1.0f.

test/libgit2.jl

@@ -1905,7 +1905,8 @@ mktempdir() do dir
  deserialize(f)
  end
  @test err.code == LibGit2.Error.ECERTIFICATE
- @test startswith(err.msg, "The SSL certificate is invalid")
+ @test startswith(lowercase(err.msg),
+ lowercase("The SSL certificate is invalid"))
 
  rm(errfile)