shallow.c: use '{commit,rollback}_shallow_file'

shallow.c: use '{commit,rollback}_shallow_file'

In bd0b42aed3 (fetch-pack: do not take shallow lock unnecessarily,
2019-01-10), the author noted that 'is_repository_shallow' produces
visible side-effect(s) by setting 'is_shallow' and 'shallow_stat'.

This is a problem for e.g., fetching with '--update-shallow' in a
shallow repository with 'fetch.writeCommitGraph' enabled, since the
update to '.git/shallow' will cause Git to think that the repository
isn't shallow when it is, thereby circumventing the commit-graph
compatibility check.

This causes problems in shallow repositories with at least shallow refs
that have at least one ancestor (since the client won't have those
objects, and therefore can't take the reachability closure over commits
when writing a commit-graph).

Address this by introducing thin wrappers over 'commit_lock_file' and
'rollback_lock_file' for use specifically when the lock is held over
'.git/shallow'. These wrappers (appropriately called
'commit_shallow_file' and 'rollback_shallow_file') call into their
respective functions in 'lockfile.h', but additionally reset validity
checks used by the shallow machinery.

Replace each instance of 'commit_lock_file' and 'rollback_lock_file'
with 'commit_shallow_file' and 'rollback_shallow_file' when the lock
being held is over the '.git/shallow' file.

As a result, 'prune_shallow' can now only be called once (since
'check_shallow_file_for_update' will die after calling
'reset_repository_shallow'). But, this is OK since we only call
'prune_shallow' at most once per process.

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>