merging (with conflicts)

[extrawurst]

this will be a multistep process:

• non-ff merging will put the repo in conflicted state (<<<< in the files)
• user resolved conflicts outside of gitui
• now we provide means to put repo into resolved state
• committing from this state should auto fill the commit msg with the typical merge commit text
• at any step we need to provide means to abort/revert the ongoing merge

[extrawurst]

turns out git2-rs lacks support for: git_repository_mergehead_foreach (issue: rust-lang/git2-rs#705)

we need that to be able to figure out which branch is currently about to be merged

[extrawurst]

update:

1. we now can merge in arbitrary branches from the branch list
2. we can undo a ongoing merge state (revert/reset, cleanup repo)

what's left to do:

• asking to do a merge commit right away if no conflicts
• helping with the typical merge commit msg if manually committing the merge

[extrawurst]

ok update time:

1. we can commit a merge now
2. we make use of standard MERGE_MSG file to repopulate merge commit msg

that leaves us with only a few small todos:

• do ff merge if possible and desired
• remove limitation that a pull has to be conflict free

[extrawurst]

closed with 8767642