This operation has quadratic complexity, which is especially painful
on Windows, where shell scripts are *already* slow (mainly due to the
overhead of the POSIX emulation layer).
Let's reimplement this with linear complexity (using a hash map to
match the commits' subject lines) for the common case; Sadly, the
fixup/squash feature's design neglected performance considerations,
allowing arbitrary prefixes (read: `fixup! hell` will match the
commit subject `hello world`), which means that we are stuck with
quadratic performance in the worst case.
The reimplemented logic also happens to fix a bug where commented-out
lines (representing empty patches) were dropped by the previous code.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
3810bd4bdd