apply: avoid leaking abandoned git-header state

When find_header() sees a "diff --git" line, it calls parse_git_diff_header() to parse the git-style extended header. That parser updates the caller's struct patch as it goes, filling in the default name, old/new names, and new/delete state. But not every "diff --git" line found while scanning is ultimately accepted as the patch header. If parse_git_diff_header() returns a length that covers only the "diff --git" line, find_header() continues scanning for another header. In that case the partially parsed git-header state must not interfere with the later traditional "---" / "+++" header. Leaving that state behind can combine incompatible metadata from the abandoned git header and the later traditional header. For example, after: diff --git a/foo b/foo --- /dev/null +++ b/foo @@ -0,0 +1 @@ +x the abandoned git header can leave an old name in the patch, while the traditional header marks the patch as creating a new file. That impossible state later trips the check_preimage() assertion that a creation patch should not have a preimage. Parse a candidate git header into a temporary patch and line number. Commit that temporary state to the real patch only when the git header is actually accepted; otherwise release it and keep scanning with the original patch state unchanged. Also reject an empty parsed default name from the "diff --git" line. An empty patch->def_name is not a valid pathname, and should not be used later as a fallback when old_name and new_name are missing. Add regression tests for both the empty default-name case and the non-empty abandoned-header case above. Co-authored-by: Mahya SamDaliri <ms3539@njit.edu> Signed-off-by: Mahya SamDaliri <ms3539@njit.edu> Co-authored-by: Haotian Zhang <haotian.zhang@njit.edu> Signed-off-by: Haotian Zhang <haotian.zhang@njit.edu> Co-authored-by: Martin Kellogg <martin.kellogg@njit.edu> Signed-off-by: Martin Kellogg <martin.kellogg@njit.edu> Signed-off-by: Zephyr Yao <zhihao.yao@njit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>