Fixes to the sage-rebaseall.sh script (followup to #20986)

[embray]

In #20986 I made some "enhancements" to the sage-rebase*.sh scripts that are used on Cygwin for DLL rebasing. While sage-rebase.sh works fine, the sage-rebaseall.sh script was left with two fatal defects:

• An outright syntax error in bash
• Because it is now a wrapper around sage-rebase.sh it will not work unless sage-rebase.sh is invoked with dash as opposed to bash (dash is a stripped-down version of bash with no dependencies on external libraries that is shipped with Cygwin--it is needed when running rebaseall since that updates all DLLs in Cygwin, including the Cygwin DLL itself).

[embray]

New commits:

​e414a91	Fix stray quote in sage-rebaseall.sh
​b5dbb48	the sage-rebase.sh script should use dash, not full bash; otherwise rebaseall will refuse to run

[charpent]

On a Virtulbox running Windows10 professional, I compiled 8.0.rc0 + #21399 + #23339 (as suggested in ​the Wiki) + #23097 + #21233 (present ticket) with the options :

export PREREQ_OPTIONS=--with-blas=atlas
export SAGE_ATLAS_LIB=/usr/lib
export MAKE="make -j4"

This passes ptestlong with no failures.

==> positive_review

Note : I have no advice on the contents of the patches (I do not know Cygwin well enough to undetstand what they are supposed to do) ; I just checked that this leads to a functional Sage.

[chapoton]

I now see that every time I build sage:

Cleaning up stale installed files....
Finished cleaning, time: 0.00 seconds.
if [ "$UNAME" = "CYGWIN" ]; then                         \
    sage-rebase.sh "$SAGE_LOCAL" 2>/dev/null;            \
fi

real	0m23,710s
user	0m4,592s
sys	0m0,712s

This is probably harmless, but quite ugly.

[embray]

That would be from #15423, not this ticket.

I agree, it's ugly. Maybe it should be prefixed with a @.