allow command like "ccache gcc"
use c++ compiler to link when c++ abi is enabled
use shallow clone in git gem
remove explicit use of g++
Merge pull request #1777 from take-cheeze/command_improvement

Command improvements.
Add ssh protocol support for bitbucket.
add tests for String#*

add test cases for 0 and -1
Merge pull request #1779 from akuroda/pr_test_string_times

add tests for String#*
Merge pull request #1780 from monaka/pr-add-ssh-protocol-support-for-bitbucket

Add ssh protocol support for bitbucket.
Merge branch 'master' of https://github.com/mruby/mruby into fix_memory_leak_in_mruby-random