Add three functions for commit history operations.

This three functions are GetCommitHistoryFirst, GetCommitHistoryNext, CommitHistoryRemove.

GetCommitHistoryFirst() get the first commit history by giving the phrase.
GetCommitHistoryNext() get the next commit history by giving the phrase.
CommitHistoryRemove() remove all the matching phrase from the commit history.
New function to export commit history to json file
Add commit-history-private.h to Makefile.am
Change column order of table commit_history.
New API: chewing_commit_history_export
AddCommitHistory is now a void function.