tmt add gives spurious errors when adding non-existent branch, with rerere enabled

[benclifford]

When adding a non-existent branch, git merge fails during materialise.

But with rerere enabled, a merge failure doesn't cause a materialise failure directly.

tmt asks rerere if there are any unresolved merge conflicts (which there aren't - because a merge didn't happen at all), and then tries to git commit the rerere-resolved conflicts that it assumes exist and are committable - but they do not exist.

So materialise fails, as desired, but it fails due to that git commit failing after a chain of unnecessary and confusing attempts at doing something.

Sample output:

benc@fitzroy:~/parsl/src/parsl$ tmt add benc-factor-launch
+ git config rerere.enabled
tmt: temporary merge tool
tmt: Reading context
tmt: Loaded context is: reload-configs-#549, master, mpi_executor_heartbeat, benc-doc-fiddles, benc-filescheme, origin/benc-executor-label, benc-apperrors-apptimeout, benc-apperrors-deserialisation, benc-apperrors-bad-formatting, benc-mypy-550, benc-mypy, benc-remove-timeout
tmt: Adding branch benc-factor-launch
tmt: Materialising context: reload-configs-#549, master, mpi_executor_heartbeat, benc-doc-fiddles, benc-filescheme, origin/benc-executor-label, benc-apperrors-apptimeout, benc-apperrors-deserialisation, benc-apperrors-bad-formatting, benc-mypy-550, benc-mypy, benc-remove-timeout, benc-factor-launch
+ git checkout --detach reload-configs-#549
Warning: you are leaving 5 commits behind, not connected to
any of your branches:

  e40536d3 tmt: merging in benc-remove-timeout
  188d5e9a tmt: merging in benc-mypy
  fc865e35 tmt: merging in benc-mypy-550 -- attempted rerere fix
  39f7498c tmt: merging in benc-apperrors-apptimeout
  4f8bf2dd tmt: merging in master

If you want to keep them by creating a new branch, this may be a good time
to do so with:

 git branch <new-branch-name> e40536d3

HEAD is now at 97cecabe... Correct reference to DataManager, not DataFlowKernel
Your branch is up-to-date with 'origin/reload-configs-#549'.
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in master' master
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in mpi_executor_heartbeat' mpi_executor_heartbeat
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-doc-fiddles' benc-doc-fiddles
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-filescheme' benc-filescheme
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in origin/benc-executor-label' origin/benc-executor-label
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-apperrors-apptimeout' benc-apperrors-apptimeout
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-apperrors-deserialisation' benc-apperrors-deserialisation
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-apperrors-bad-formatting' benc-apperrors-bad-formatting
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-mypy-550' benc-mypy-550
tmt: Merge failed
tmt: Merge stdout:
Auto-merging requirements.txt
CONFLICT (content): Merge conflict in requirements.txt
Auto-merging parsl/data_provider/files.py
Auto-merging parsl/__init__.py
Automatic merge failed; fix conflicts and then commit the result.

tmt: Merge stderr:
Resolved 'requirements.txt' using previous resolution.

+ git rerere remaining
tmt: git rerere reports no remaining conflicts, so committing
+ git commit -a -m 'tmt: merging in benc-mypy-550 -- attempted rerere fix'
[detached HEAD a90f5ac1] tmt: merging in benc-mypy-550 -- attempted rerere fix
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-mypy' benc-mypy
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-remove-timeout' benc-remove-timeout
+ git config rerere.enabled
+ git merge --no-ff -m 'tmt: merging in benc-factor-launch' benc-factor-launch
tmt: Merge failed
tmt: Merge stdout:

tmt: Merge stderr:
merge: benc-factor-launch - not something we can merge

+ git rerere remaining
tmt: git rerere reports no remaining conflicts, so committing
+ git commit -a -m 'tmt: merging in benc-factor-launch -- attempted rerere fix'
HEAD detached from refs/heads/reload-configs-#549
Untracked files:
[...]
nothing added to commit but untracked files present
tmt: callCommand: git commit -a -m 'tmt: merging in benc-factor-launch -- attempted rerere fix' (exit 1): failed