util: enhance `nullcontextmanager` to be able to return __enter__ result
This makes its functionality and signature equivalent to Python 3.7’s
contextlib.nullcontext().
$ hg init
$ mkdir d1 d1/d11 d2
$ echo d1/a > d1/a
$ echo d1/ba > d1/ba
$ echo d1/a1 > d1/d11/a1
$ echo d1/b > d1/b
$ echo d2/b > d2/b
$ hg add d1/a d1/b d1/ba d1/d11/a1 d2/b
$ hg commit -m "intial"
Test single file
# One recorded copy, one copy to record after commit
$ hg cp d1/b d1/c
$ cp d1/b d1/d
$ hg add d1/d
$ hg ci -m 'copy d1/b to d1/c and d1/d'
$ hg st -C --change .
A d1/c
d1/b
A d1/d
# Errors out without --after for now
$ hg cp --at-rev . d1/b d1/d
abort: --at-rev requires --after
[255]
# Errors out with non-existent source
$ hg cp -A --at-rev . d1/non-existent d1/d
d1/non-existent: no such file in rev 55d1fd85ef0a
abort: no files to copy
[255]
# Errors out with non-existent destination
$ hg cp -A --at-rev . d1/b d1/non-existent
abort: d1/non-existent: copy destination does not exist in 8a9d70fa20c9
[255]
# Successful invocation
$ hg cp -A --at-rev . d1/b d1/d
saved backup bundle to $TESTTMP/.hg/strip-backup/8a9d70fa20c9-973ae357-copy.hg
# New copy is recorded, and previously recorded copy is also still there
$ hg st -C --change .
A d1/c
d1/b
A d1/d
d1/b
Test moved file (not copied)
$ hg co 0
0 files updated, 0 files merged, 2 files removed, 0 files unresolved
$ mv d1/b d1/d
$ hg rm -A d1/b
$ hg add d1/d
$ hg ci -m 'move d1/b to d1/d'
created new head
$ hg cp -A --at-rev . d1/b d1/d
saved backup bundle to $TESTTMP/.hg/strip-backup/519850c3ea27-153c8fbb-copy.hg
$ hg st -C --change .
A d1/d
d1/b
R d1/b
Test using directory as destination
$ hg co 0
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cp -R d1 d3
$ hg add d3
adding d3/a
adding d3/b
adding d3/ba
adding d3/d11/a1
$ hg ci -m 'copy d1/ to d3/'
created new head
$ hg cp -A --at-rev . d1 d3
abort: d3: --at-rev does not support a directory as destination
[255]