[GSoC] [RFC] Proposal: Teach git stash to handle unmerged index entries.

[Kapil Jain <jkapil.cs@gmail.com>]

Plan to implement the project.

Objective:
Teach git stash to handle unmerged index entries.

Description:
When the index is unmerged, git stash refuses to do anything. That is
unnecessary, though, as it could easily craft e.g. an octopus merge of
the various stages. A subsequent git stash apply can detect that
octopus and re-generate the unmerged index.


Implementation Idea:
Performing an octopus merge of all `stage n` (n>0) unmerged index
entries, could solve the problem, but

What if there are conflicts in merging ?
In this case, we would store(commit) the conflicted state, so they can
be regenerated when git stash is applied.

How to store the conflicted files ?
create a tree from the merge using `git-write-tree`
and then commit that tree using `git-commit-tree`.


Relevant Discussions:
https://colabti.org/irclogger/irclogger_log/git-devel?date=2019-04-05#l92
https://colabti.org/irclogger/irclogger_log/git-devel?date=2019-04-09#l47


Idea Execution Plan: Divided into 2 parts.

Part 1: Store the unmerged index entries this part will work with `git
stash push`

stash.sh: file would be changed to accommodate the below implementation.

Step 1:
Extract all the unmerged entries from index file and store them in a
temporary index file.

read-cache.c: this file is responsible for reading index file,
probably this implementation will end up in this file.

Step 2:
cache-tree.c: study and implement a slightly modified version of the
function `write_index_as_tree()`

int write_index_as_tree(struct object_id *oid, struct index_state
*index_state, const char *index_path, int flags, const char *prefix);

this function is responsible for writing tree from index file.
Currently in this function, the index must be in a fully merged state,
and we are dealing with its exact opposite. So a version to write tree
for unmerged index entries will be implemented.

Step 3:
write-tree.c: some possible changes will go here, so as to use the
modified version of write_index_as_tree() function.

Step 4:
use git-commit-tree to commit the written tree and store the hash in
some file say `stash_conflicting_merge`

Step 5:
Write tests for all implementation till this point.

Part 2: Retrieve the tree hash and regenerate the state of repository
as it was earlier.

Step 6:
Modify implementation of `git stash apply` for regenerating the committed tree.

Step 7:
Write tests.