rebase -i: drop, missing commits and static checks

Changes between versions:

In t3404:

Changed 'test_rebase_end' to 'rebase_setup_and_clean'. Changed the indentation in 'rebase_setup_and_clean'. Changed the names of the branches created in my tests (avoid names like 'tmp'). Added 'test_might_fail' in front of 'git branch -D'. Remove 'test_config rebase.missingCommitsCheck error' in the last test ('static check of bad SHA-1') because it was useless.

In git-rebase--interactive.sh:

Errors found in commands and SHA-1 (static check) are displayed on the
spot.
I used return values to signal to the calling functions if there is an
error.
The whole while block in 'check_bad_cmd_and_sha' with the return is in a 
'(
	[...]
)'
block because 'retval' would not have been correctly affected when
getting out of the loop since the while is in a pipe.

A thought occured to me: Shouldn't all the checking also be called in a 'rebase --continue', considering that it can be called after a 'rebase --edit-todo' ? (Right now it is only called after closing the editor in 'rebase -i')

[PATCHv7 1/3] git-rebase -i: add command "drop" to remove a commit [PATCHv7 2/3] git rebase -i: warn about removed commits [PATCHv7 3/3] git rebase -i: add static check for commands and SHA-1

Rémi (It seems that with 'send-email', 1 time out of 2 I get: '5.7.8 Error: authentication failed: authentication failure')