[PATCH v2 2/2] Handle more file writes correctly in shared repos

In shared repositories, we have to be careful when writing files whose permissions do not allow users other than the owner to write them.

In particular, we force the marks file of fast-export and the FETCH_HEAD when fetching to be rewritten from scratch.

This commit does not touch the following users of fopen() that want to write files:

- git am, when splitting mails (git-am correctly cleans up its directory
  after finishing, so there is no need to share those files between users)

- git apply, when writing rejected hunks

- git fsck, when writing lost&found blobs

- git merge-file, when writing merged files (when Git itself calls
  merge-file, the file in question was already there, with shared
  permissions).

- git submodule clone, when writing the .git file, because the file will
  not be overwritten

- git_terminal_prompt() in compat/terminal.c, because it is not writing to
  a file at all

- git diff --output, because the output file is clearly not intended to be
  shared between the users of the current repository

- git fast-import, when writing a crash report

- mailinfo() in mailinfo.c, because the output is clearly not intended to
  be shared between the users of the current repository

- check_or_regenerate_marks() in remote-testsvn.c, because this is only
  used for Git's internal testing

- git rerere, when writing resolved files, because the files in question
  were already written with the correct permissions

Note that this patch does not touch callers of write_file() and write_file_gently(), which would benefit from the same scrutiny as to usage in shared repositories. Most notable users: branch, daemon, submodule & worktree, and a worrisome call in transport.c when updating one ref (which ignores the shared flag).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/fast-export.c | 2 +-
 builtin/fetch.c       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)