merge-one-file: fix broken merges with alternate work trees

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 makes merge-one-file chdir to the toplevel of the working tree (and exit if we don't have one). This most closely matches the assumption made by the original script (before separate work trees were invented), and matches what happens when the script is called as part of a merge strategy. 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> Signed-off-by: Junio C Hamano <gitster@pobox.com>