Fixed issue #874: Add --rebase option to pull dialog

"--rebase" option is introduced by 25bd4c961830590ffdb51202e935fd168b7b2484,
but further working is needed.
So, the "Launch Rebase After Fetch" checkbox is hidden from Pull dialog.

For the conflict reason,
the Rebase dialog uses cherry pick(instead of "--rebase" option)
to perform rebase function.
And CLI option "--rebase" should not be used. (never and ever)

Therefor, if "Launch Rebase After Fetch" checkbox is checked in Pull dialog,
then "git.exe pull --rebase" is adapted to "Fetch & Rebase".

Also support ```branch.<branch_name>.rebase```
Once the branch.<branch_name>.rebase = true,
"Launch Rebase After Fetch" checkbox will be checked and disabled.

Other reference:
Please see branch.autosetuprebase
in https://www.kernel.org/pub/software/scm/git/docs/git-config.html

Signed-off-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
no. 
let see what happen:
1. rebase = true in config
2. open Pull dialog
3. user un-check rebase, so m_bRebase = false
4. the ```Fetch Rebase```(DoFetch()) will not be called
5. and ```git.exe pull``` is execute without "--rebase", but rebase will be invoke.