[PATCH 2/2] merge-one-file: fix broken merges with GIT_WORK_TREE

The merge-one-file tool predates the invention of GIT_WORK_TREE. By the time GIT_WORK_TREE was invented, most people were using the merge-recursive strategy, which handles resolving internally. Therefore these features have had very little testing together.

For the most part, merge-one-file just works with GIT_WORK_TREE; most of its heavy lifting is done by plumbing commands which do respect GIT_WORK_TREE properly. The one exception is a shell redirection which touches the worktree directly, writing results to the wrong place in the presence of a GIT_WORK_TREE variable.

This means that merges won't even fail; they will silently produce incorrect results, throwing out the entire "theirs" side of files which need content-level merging!

This patch properly prefixes the path in the shell redirection. An alternative strategy would be to have merge-one-file use git-sh-setup, and cd_to_toplevel beforehand. I opted for the minimal fix here, as changing directories would impact where merge-one-file is unpacking and working with files.

While we're at it, we'll also error-check the call to cat. Merging a file in a subdirectory could in fact fail, as the redirection relies on the "checkout-index" call just prior to create leading directories. But we never noticed, since we ignored the error return from running cat.

Signed-off-by: Jeff King <peff@peff.net>
---
 git-merge-one-file.sh  |    4 +++-
 t/t6060-merge-index.sh |    2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)